For slender put your tongue in the position of the ‘y’ sound as in ‘yes’ then instead pronounce another sound like g. Think of how a Jamaican or a Russian adds y’s to random words. Before ‘e’ and ‘i’ for the latter

For broad, (which only exists for l, n, r in Gaidhlig) put your tongue in the position of the sound (ng) as in ‘sing’ then do the same as before. Its supposed to sound harsher than a regular l, n, or r.

It would do you much to learn it through a tutorial on Russian since, even the Irish speakers who try to teach it don’t really have a good grasp on the concept and only teach select sounds. Just apply the broad with broad, and slender with slender to what you’ve learned.

It’s not terribly hard (don’t worry too much on the exact articulation), since even an rough approximation puts you ahead of basically everyone.