My first step suggestion would be saying "d" instead. This is actually what's taught in choral singing diction, for instance -- since not everyone call roll their r's, but we all have to sing precisely the same vowels and consonants, we're told to say "d" if you don't know how to roll r's.
It seems a strange idea -- until you try it. It's actually exactly the same little tongue flip. The only difference is when you want to do a LONG rolled r (as opposed to a quick flip, which is actually what is used in many languages most of the time -- the long r is saved for emphasis).
To do the long rolled r, you kind of fight between blowing the tip of the tongue off the alveolar ridge (behind the teeth) with your breath, and keeping the tip up with your tongue muscles. Exhalation vs. muscles trying to keep the tip up. So it blows down, pushes back up, blows down, pushes back up, etc. That's what it feels like to me, anyway.
The "l" sound coming out instead can mean that the tongue is spread too wide, engaging the sides of the mouth -- it needs to be more tightly held in, and the mouth open enough on the inside. It can also mean that it's not the tip of the tongue engaging, but the area just behind the tip.
It can also just be not finding the exact right pressure -- it's a resonance frequency thing. If you don't use enough pressure, it just goes "dssssssssthhhhhh"... too much pressure and you get "TST.... TSTH..." That part just takes practice.
My grandmother was british and she taught us all to roll our r's in the good queen's english when we were very young. I think she had us start with the tips overcurled towards the back, so that when you start exhaling it moves forward into place, already vibrating... rather than starting with the tip already firmly behind the teeth.
I remember those practice sessions with her very fondly.
And I credit her entirely for my lovely rolled r's.