Open your desired paper

Go to Image>Canvas Size> Resize your paper to 500x500 (or size of your main canvas)

Open your mask you want to use

Select your paper again

Go to Layers>New Mask Layer>From Image

A new window should open

Under Source Window select the name of your mask and click ok

Your mask should apply to your paper now

Go over to your Layers window on the right side of your PSP

right click your paper, scroll down to merge>Merge group

Now you Copy>Paste>Paste as new layer

Move this down to make it your bottom layer

You can scale or resize if you need