For me it’s a great part excercise, and training, and knowledge of a few little tricks. And inspiration of course, and a sea of text, poetry, lyrics, experiences, whatever you have absorbed thoughout your life. Passive TV consumption is poison for creativity, read more books.
It seems somewhat hard to make real good sounding lyrics, that are catchy and just “sound” right. Same I see with poetry, where the rhythmical sense seems sometimes so lost and random for me that I hardly enjoy most contemporate stuff.
Few tricks of mine are, first making up somekind of catchy rhythm/melody in the head. You have to practice to imagine music/words like the earcatcher stuff you sometimes can’t get out of your head when you heard it in the radio for that. Yes it’s possible to do that at will, just like thinking/visualising of a picture or image. It doesn’t have to be the final stuff of the song you make the lyrics for, but similliar. If you fail at that, just listen to some song that’s similliar and hit repeat on the player, if it’s an instrumental it’s better because your lyrics won’t be so similliar to the original ones.
Then I sit down with a pen and paper and flow words along with that rhythm/melody in my head, writing down what seems ok, and rewriting most of the stuff after each line and in between until rhythm and rhymes and such sound ok. I’ve also though about quickly recording that stuff with humming and a simple beat so the idea won’t get lost between sessions, happens too often for me. I take care that the melody/rhythm is in sync and doesn’t deviate too often from a basic pattern I thought through/have built up, that makes the thing way catchier.
Also I take care with words that have multiple syllables, with those one is always accented, and this one needs to fit into the rhythm and melody of the song, or it’ll sound clumsy! My main language is german with many multi-syllable words, and there it’s probably even more important that with english. Also think about the sentence of that line as you’d speak it, and watch for which words you’d accentuate when you’d do so, and what the speaking melody of that would be. The more it matches the rhythm of the song and the melody, the catchier and “more right” it’ll sound when sung. Also I make dot/dash/line markings of the rhythm I imagine for that stuff when sung or rapped below/above each line of text, so I can analyze it better, recover the rhythm of it after forgetting my initial head-melody stuff, and check up how consistent it is.
Too much consistence can however make a song sound too much easy listening like schlager volksmusik dreck. Will probably sell well but it’ll also be boring for people who like their minds being teased.
Good practicing, even if you hate hip-hop, ist writing silly raps, as you won’t need to care about melody, but just about accented syllables and basic rhythm consistency.
Also…inspiration…I seldom think of a concept beforehand, I just have halve a bottle of wine an a rhythm/melody concept in my head I jam about and think of a simple (nonsensical) line that fits in, then nail it down to paper and just go on by rhymes and rhythm, shaping up a sense I hadn’t in mind at the beginning, maybe even replacing the original first line(s) if they don’t fit anymore. Think of it not like a flowing that has to be right immediately, but like a clay sculpture you add, remove, and change parts in a long process until you’re satisfied and proportions are right. With a lot of practice you’ll get good “shapes” much faster, of course, and might even be able to improvise decent stuff at some point.
Hope that helps making better lyrics 
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And she is great at writing lyrics. She can improvise something in 1 take and it will be 100x better than me sitting down staring at a pad for 4 hours and scratching out bad ideas.
