Here is a fairly easy and very useful one.
Let's say I've created a character, who is wearing jeans and I want put some denim texture into his jeans.
In Photoshop I open a New Layer
I use the Pen Tool to draw the shape of his jeans.
Then I select the Path Selection Tool (A).
I set the Foreground Colour to the one I want.
I right click on the jeans shape and select Fill Path.
Foreground Colour.
I have found a denim texture online.
I open the denim in a new Photoshop document.
I go to Image > Adjustments > Desaturate to take the colour out of it.
I use the Rectangular marquee Tool to select it.
I go to Edit > Copy, and then I Edit > Paste it into the document with my trousers in it.
The Layers menu should now look like this:
With the Denim layer highlighted I press Command T and resize the denim image to the required dimensions, and the hit RETURN.
With the denim layer highlighted, I hold down the Command key and click in the ICON of the trousers in the Trousers layer.
It should look like this:
I then click on the Add Layer Mask icon.
And my Image should look like this:
I have created a mask in the shape of the jeans, which is filled with the denim pattern.
