If the sub is on the instance you’re logged-in to (lemmy.works for you, I assume), then you just need to expand the “Sidebar” tab and tap “Subscribe”.

If it’s on another instance, you have to navigate to https://lemmy.world/c/sub@instance. So the main sub for sh.itjust.works would become (https://lemmy.world/c/main@sh.itjust.works)[https://lemmy.world/c/main@sh.itjust.works] THEN go into the sidebar and the “Subscribe” button will be there. I think of it as just using a different door to get to the same content. And at this door you know the bouncer.

Alternatively, the sorta short linked version within lemmy would be !sub@instance . So that would look like (!main@sh.itjust.works)[!main@sh.itjust.works]. You can type that into the search from your main instance and the first result should be a link. That link will take you to a version of the sub with the same “Subscribe” option under the sidebar tab. Hope this helps!