Step 1: Start with a clean working directory and perform git status to see if there's any pending modification, deletion, or addition.
Step 2: Select the commit(s) you want to split. Here's a guide to split different commits:
Splitting the most recent commit:
git reset HEAD~a
Okay, now commit the pieces individually in the usual way, producing as many commits as you need.
Splitting a commit farther back:
This requires rebasing. Say, it's only 3 commits back, then run the following command,
git rebase -i HEAD~3
If it's farther back in the tree, than you want to count, then
git rebase -i 123abc~
Here, 123abc is the SHA1 of the commit you want to split up.
For more information please go through the following tutorial to get more info about git: