How do you merge in GIT on Windows?

I suggest kdiff3. Install it and put something like:

[merge]
        tool = kdiff3
[mergetool "kdiff3"]
        path = c:/Program Files (x86)/KDiff3/kdiff3.exe
[diff]
        tool = kdiff3
        guitool = kdiff3
[difftool "kdiff3"]
        path = c:/Program Files (x86)/KDiff3/kdiff3.exe

in your .gitconfig

To edit the global config (.gitconfig): git config --global --edit

The installation path on your machine may be different from c:/Program Files (x86)/KDiff3/kdiff3.exe. Replace that string above with actual path to your kdiff3 binary. And don’t forget you have to use forward slashes, /, and not backward ones (\) like native programs usually do.