If you're up against a legitimate grappler in MMA, the only consistent way to really secure them is to explode into it and catch your opponent completely off-guard in a transition. Here's my favourite one from Charles Oliveira vs Darren Elkins:
Methodically working towards a triangle choke from your back is not only really difficult and dangerous due to the inherent threat of getting your face elbowed, but also strategically dim, since guys can literally just back away as soon as you open your guard.
You would have to be a master of utilizing the rubber guard and that type of stuff to eliminate those factors, and even then, the guy on top can just stall and wait for a referee stand-up and you have to reset.