It’s not a direct conversion as LUFS uses frequency-dependent weighting. So for the same dBFS, you could get different LUFS depending on the content.

The best way to measure is to use a so-called “loudness meter”. Most DAWs have one native, and most mastering plugins provide one as well for convenience. But if you need one for some reason, this free one from GoodHertz is pretty nice.

LUFS is usually given in three readings: Momentary, Sustained, and Integrated. The rule of thumb is, loop the loudest part of your song. Reset the integrated meter. Twiddle your master level until the integrated reads -14 LUFS (resetting the meter between tweaks). Then you’re done.

If you want to learn more, this SOS article is a decent jumping off point.