The garage thing is the git repository, it is nice to maintain source code.
But you are looking for the extra package repository.
So first you need to ask for extras access with you garage account.

Then you need to be maintainer of the package, this could be asked with logged in garage account on the package page. When nothing happens you need to ask for it in our irc channel.

When you are maintainer of a package, then you can promote packages from devel to testing, only when you are logged in with garage account.

Before you can promote your updated package, the source code needs to be uploaded to extras-devel with ssh or with the extra assistant.

When you want to promote a package from testing to extras you need to have enough votes to promote.

Attached screen shots for orientation. The second one is edited to have a clue where the promote link should be.
Because I am no maintainer for this package and can't see this link and anyway it is already promoted because same version is in all repos.