There are a couple tools, lemmy-helper https://github.com/RocketDerp/lemmy_helper/tree/master has a command line tool and there is also a simpler "Lemmy Community Seeder": https://github.com/Fmstrat/lcs

Here's an Userscript that I'm working that has a slightly easier method, it provides a button on remote instances that sends you to the screen you can click to go to the community from your instance (so you dont have to manually type the !somecommunity@somewhere.ml) on your search.

It makes it slightly easier. Works on mobile and desktops in browser.