There is multiple ways but I only know a few..
One way is to search up your name and sometimes it can come up with results of your skin on images and you can save/crop the picture so it will fit to the avatar.
The other way is to go onto http://minecraft.novaskin.me and upload your skin and once again crop it to the avatar size.
If you look in the top right corner you will see a zoom button, looks like this + -
Zoom into your face and use the print screen button on your keyboardist should screenshot your face and you will be able to locate it where ever it saves it to
Then from there crop it in paint and upload it to your avatar
Let me know if you want more help