From your reference image of choice, make a selection around the eye, copy, and paste it into your texture work file. Right-Click anywhere in the image and choose "Free Transform" to be able to quickly scale and rotate etc. the pasted image.

Move and scale the pasted eye so that it lines up with the UVs of the character's head. Next go back to your reference image and make a selection around the nose, copy, and paste it into your work image.

I copy each piece of the face seperating because it gives me more control over scaling and moving those parts to line up properly to the UVs. It's rare that you'll be able to copy an entire face onto your UVs and have everything line up and be the right size, right off the bat.

While doing this, Save your image and go back to 3dsmax to check on the placement of things. It should refresh the texture automatically every time you save your PSD file.

Do the same thing for the mouth and check your progress in Max. Make sure that the mouth actually falls in the correct parts of the geometry on the 3D model. If it goes too far past the lip or doesn't line up with the crease between lips, bo back into photoshop and scale/move things around some more.

Once the majority of the face is down, I copied the side angle of the head and scaled it so it was approximatly the correct size and shape. Now to open one of photoshop's most helpful tools when making textures from photo reference....

Liquify

Go to Filter > Liquify... (Ctrl+Shift+X) and wait a moment for it to load (it always takes a bit).
First thing you'll want to do is check the Show Backdrop option, Use: All Layers, and Mode: Behind.

The image below has all of the important areas circled. You'll just use the default tool, but you can feel free to experiment with the others. The great thing about Liquify is that it lets you nudge around things without bluring the image. The brush size is really sensive. Change the brush size as needed and move stuff around.

When you're done in Liquify, just click OK and it'll update your main work image in photoshop. Save and check stuff in 3dsmax to see if anything needs nudging in different locations. I did this and I saw that the ears were still a little messed up, and the hair line needed some work.

So I went back to photoshop, did Liquify again, and adjusted the shape and alignment of the hairline so that it'd display better on the actual model.

Okay, now its time to clean up some of these overlaps. Often times, using the Erasor with a fuzzy brush, and going over the edges of each layer is enough to blend things together quite nicely. If you need to, you can use the Clone Stamp Tool as well.

And another save and check in 3dsmax. Everything seems lined up good. The forehead still needs some work, but I can clean that up later when I start doing manual touch-ups. I decided that the eye layer could probably be stretched a little bit in width (eyes looked a little small anyways) and it'd fill that annoying gap without having to mess with the clone stamp tool.