Let's see...I'm using Paint Shop Pro (PSP) 6, so it will be a little different, but all the tools are there.
First, make sure both pics are "views" with the same "zoom" level on PSP. PSP can sometimes default to 50% or so zoom. So whether you use the VIEW drop down menu, or the wheel of the wheel mouse when your mouse is over the title bar of the picture.
After you have them on the same zoom level, put them side by side and resize one to match the other. Shrinking the larger is usually what I do and I'll choose IMAGE/RESIZE and make it 85% of what it was...then do it again, etc.
If at any point a step doesn't work as planned used EDIT/UNDO (CTRL-Z) to go back a step. I'm constantly doing that.
Once they are actually sized and zoomed the same, then study both pics to plan what you want to cut out and paste. You may even want to darken or lighten one picture at this stage.
Once you have an idea where to cut, zoom on the "face" picture to larger than normal. It's takes time to see what works well with you, your mouse, your mousepad size, etc. The more you zoom in, the more room you have for small mouse errors, but the more you'll have to move your mouse.
Use the "lasso" tool to select the desired area. Sounds easy, right? I usually get it on the 35th try...
Once it's selected, copy it using EDIT/COPY or CTRL-C.
Then move to the target picture and "paste as new selection" or CTRL-E. You can position it just right, rotate it a little, even "skew" it if needed.
Then you start blending since there's an obvious line where you pasted the face (plus choosing natural breakpoints such as the jawline or the neckline of clothing). Zoom in the picture to several times it's original size and using COPY (CTRL-C) and PASTE (CTRL-E) you can copy tiny patches of colors pasting them over the right spots to give certain areas a little shading. Very tedious, but similar to when a doctor does skin grafts, you're doing "color-grafts."
Once you spend forever on that, there's two tricks to help a bit more.
#1 - shrink the picture a little bit. Say only 5-15%, but the computer will reblend it a little more.
#2 - Export to JPEG picture and don't use the best quality. You don't have to have the worst quality, but the computer will again reblend the pic.