I've been asked how I do this a couple of times now, so I decided to make a tutorial on it. I hope it's not impossible to follow due to my horrible English in this tutorial :v

First up, Create all the parts of the texture, apart from the square that'll be the "knuckles" on the hand. (Highlighted in the picture)



Select that part and copy it. Then, paste it in a new canvas and go to View > New Guide... and type in these variables:



You should now have this:



Now, -this is the tricky part- go to the main l or r_hand texture and copy the part of the front of the hand that touches the "knuckles"
Now, paste that part in the canvas with the guidelines you created earlier.

Now you know where the sides of the hand touch the outer part, use this to create a texture that touches those parts.
When you're done, simply paste the new texture back into the l_hand or r_hand image and save.
The result should be something like:




Now go enjoy your new hand texture 8D

FYI, that particular section of the image is 20% off the entire picture. Place your guides accordingly as well =)

I never tried orientation. I always slice my image.