Branch support is all done (modulo a bug about the behaviour when the branch is closed).
However, the support in snapcraft for branches will be released in 2.30, so maybe you should wait for it to use it.
You can release into branches today, but it will not show them properly in the Store responses.
To release into a branch or close a branch you just include it in the channel, example:
snapcraft release mysnap 123 stable/hotfix
To install from a branch, it’s similar (also working now):
snap install mysnap --channel=stable/hotfix