Download and install the Git command line extension. You only have to set up Git LFS once.
git lfs install
Select the file types you'd like Git LFS to manage (or directly edit your .gitattributes). You can configure additional file extensions at anytime.
git lfs track "*.psd"
Make sure .gitattributes is tracked
git add .gitattributes
There is no step three. Just commit and push to GitHub as you normally would.
git add file.psd
git commit -m "Add design file"
git push origin master