Which way to merge with git?

Say I have two branches

master -- A -   -   -   -   -  - merge
          \                    /
           \- develop -- B -- C

Now if I want to merge it will be a fast forward, but should I do

git checkout develop
git merge master

or

git checkout master
git merge develop

And what if I have possible conflicts

master -- A - D -  -  -  -  -  -merge
          \                   /
           \- develop -- B -- C

Should I now merge in to develop or into master? This is a bit confusing, so a good explanation would be really appreciated