I decided to write up this tutorial after seeing this question asked for the 1000th time and no really easy tutorials on it.
Open your template. For the sake of this tutorial I will assume it is a layered PSD file, but will discuss how to do it without a layered psd file at the end of the tutorial.
Once your template is open you will see all the various layers in your layer palette. We are going to start with one of the elements in our template. In my template I have a star your template may have whatever, but anyhow this is how you do it.
Make sure you selected the star layer in your layer palette and then go to the magic wand tool to select the star.
Then click on the star layer with your magic wand tool to select it.
Leave your star still selected in your template file and then open up the paper you want to use and then choose Edit > Copy to copy it and then go back to your template and choose Edit > Paste as new layer.
You should automatically be on the new layer with your paper and star still selected, but if your not just make sure you select the paper layer in the layer palette. Then all you have to do is go to the top and choose Selections > Inverse and then hit the delete key and voila you have a star with your patterned paper!
You can then just delete the original star from the template since your done with it. Deselect your star and repeat this process with each layer in your template.
NOTE: As I stated above I said I’d mention what you had to do if you didn’t have a layered PSD file for your template. If your template comes in seperate png files you will have to create a new file and Edit > Copy, then Edit > Paste to copy each layer into the new file. You will then have to move the pieces around to get them in the right place and then follow all the rest of the steps for this tutorial.