The mesh used in this tutorial is
MFD's Straight Pants
Before you Begin
You should know there are two types of texture maps used in re-texturing a product. The first is an opacity map which is used to create transparency. Then there is the diffuse map which is what you draw your texture on.
Copy or save the second map and paste or open it in paint shop. We don't really need the first map with this mesh because the creator so kindly made everything line up perfectly!
Create Your Opacity Map
Opacity maps are pretty simple overall. Anything white you will be able to see, anything black will be hidden, and anything grey will be partially transparent.
Before you Begin
You should know there are two types of texture maps used in re-texturing a product. The first is an opacity map which is used to create transparency. Then there is the diffuse map which is what you draw your texture on.
Copy or save the second map and paste or open it in paint shop. We don't really need the first map with this mesh because the creator so kindly made everything line up perfectly!
Create Your Opacity Map
Opacity maps are pretty simple overall. Anything white you will be able to see, anything black will be hidden, and anything grey will be partially transparent.
1. Create a new layer.
A window will pop up, click ok.
Click on your pen tool 
These are the settings I use. I recommend experimenting around to see what the different things do. For photoshop users we're using bezier curves
These are the settings I use. I recommend experimenting around to see what the different things do. For photoshop users we're using bezier curves
3. Click on one point of your picture, then click somewhere else and drag. You will see that this creates a line and tha as you drag it it will curve in different directions. This will help you create smooth lines.
4. Click on your selection tool
5. Be sure that anti-aliasing is unchecked and feather is set to 0 and then click and drag across the map. You'll see little "dancing ants" when you're done.
4. Click on your selection tool
5. Be sure that anti-aliasing is unchecked and feather is set to 0 and then click and drag across the map. You'll see little "dancing ants" when you're done.
6. Click on your flood fill tool 
Click on the inside of your box to fill it with a solid white color.
7. See the little bit of black showing through? Well we don't want that otherwise our jeans will not show up there, so click on the air brush tool.
These are the settings I used. A brush size of 3 to 5 is usually good for this.
Now you may want to zoom in a little bit so you can see what you're doing. You can use your middle mouse wheel/button or the zoom tool. It looks like a magnifying glass.
Go along the part you can see through and cover it up.
Once you are done deselect your area.
8. Create a new layer again.
9. Flood fill the entire layer with black.
10. Drag the black layer underneath the white layer. (if you cannot see your layers go to View > Palettes > Layers)
11. Save your file.
Click on the inside of your box to fill it with a solid white color.
7. See the little bit of black showing through? Well we don't want that otherwise our jeans will not show up there, so click on the air brush tool.
These are the settings I used. A brush size of 3 to 5 is usually good for this.
Now you may want to zoom in a little bit so you can see what you're doing. You can use your middle mouse wheel/button or the zoom tool. It looks like a magnifying glass.
Go along the part you can see through and cover it up.
Once you are done deselect your area.
8. Create a new layer again.
9. Flood fill the entire layer with black.
10. Drag the black layer underneath the white layer. (if you cannot see your layers go to View > Palettes > Layers)
11. Save your file.
Create your Diffuse Map
This is where you'll need to practice a lot. You may not get it right the first time, don't get discouraged, just go back and try again later.
1. Hide your black layer.
2. Highlight your white layer, then reduce the opacity.
Your image should look something like this now
3. Create a new layer
4. Click on your pen tool and set the line style to "Dot" with a width of 1.
Now use your pen tool and draw all of the seams on the pants. I recommend using a tan or darker brown color. This is how mine looked.
5. Create a new layer and flood fill the entire layer with a dark color. Any color you want your jeans to be will work.
6. Use the layer palette like you did before to drag your layer underneath the layer with the seams on it.
7. This time we're going to choose a lighter color and add some texture before we flood fill. I chose a hatch texture.
Now your jeans should look something like this
8. Now we need to highlight the seams of the jeans. Select your dodge tool
These are the settings that I use, but you may find it easier to use a larger or smaller brush size. Just experiment and use what works best for you.
Now don't try to be perfect when you use this tool! We can fix up any squiggles later. Here's how mine looked.
9. Select your Burn tool. It is in the same drop down as your dodge tool, or if you've got a really steady hand you can right click and draw with your dodge tool to automatically burn.
Add wrinkles with both your dodge and burn tool. It may help you to look at a few pictures of clothing, or even look down at your own pants while you work. There's no right or wrong way to do this, it just takes practice.
10. Increase your brush size on your dodge and burn and add large areas of highlights (light areas) and shadows (dark areas).
11. Click on your smudge tool. It can also be found where your dodge and burn tools are. These are the settings that I used. It doesn't take much movement so be careful! If you find it blends too much then turn the opacity down.
Now go along and smooth out those squiggly lines. Also blend together your lighter and darker areas. It's okay if your texture gets smudged too.
12. Create a new layer, and choose "multiply" in the drop down list before you click ok.
Flood fill the new layer with a medium to dark texture. I used something that looked slightly bumpy and came up with this.
13. Finally go to your Layer Palette and turn the opacity down on the seams a little bit. You can even double click on the layer listed and change the settings to see if you can come up with a different effect you like more. Add buttons, belts or anything else you like!
The final result:
This is where you'll need to practice a lot. You may not get it right the first time, don't get discouraged, just go back and try again later.
1. Hide your black layer.
2. Highlight your white layer, then reduce the opacity.
Your image should look something like this now
3. Create a new layer
4. Click on your pen tool
and set the line style to "Dot" with a width of 1. Now use your pen tool and draw all of the seams on the pants. I recommend using a tan or darker brown color. This is how mine looked.
5. Create a new layer and flood fill
the entire layer with a dark color. Any color you want your jeans to be will work. 6. Use the layer palette like you did before to drag your layer underneath the layer with the seams on it.
7. This time we're going to choose a lighter color and add some texture before we flood fill. I chose a hatch texture.
Now your jeans should look something like this
8. Now we need to highlight the seams of the jeans. Select your dodge tool
These are the settings that I use, but you may find it easier to use a larger or smaller brush size. Just experiment and use what works best for you.
Now don't try to be perfect when you use this tool! We can fix up any squiggles later. Here's how mine looked.
9. Select your Burn tool. It is in the same drop down as your dodge tool, or if you've got a really steady hand you can right click and draw with your dodge tool to automatically burn.
Add wrinkles with both your dodge and burn tool. It may help you to look at a few pictures of clothing, or even look down at your own pants while you work. There's no right or wrong way to do this, it just takes practice.
10. Increase your brush size on your dodge and burn and add large areas of highlights (light areas) and shadows (dark areas).
11. Click on your smudge tool. It can also be found where your dodge and burn tools are. These are the settings that I used. It doesn't take much movement so be careful! If you find it blends too much then turn the opacity down.
Now go along and smooth out those squiggly lines. Also blend together your lighter and darker areas. It's okay if your texture gets smudged too.
12. Create a new layer, and choose "multiply" in the drop down list before you click ok.
Flood fill the new layer with a medium to dark texture. I used something that looked slightly bumpy and came up with this.
13. Finally go to your Layer Palette and turn the opacity down on the seams a little bit. You can even double click on the layer listed and change the settings to see if you can come up with a different effect you like more. Add buttons, belts or anything else you like!
The final result:
