Almost. I get what they mean about casting combust too early. It can be cast during another spell, so you always cast it as the other spell is almost finished. You might even he able to cast it while the spell is in the air. That way, all that time spent casting isn't letting combustion tick down unused. Also, you want to precast pyroblast if you can to get a bigger ignite ticking (not as easy in dungeon runs).
Precast pyroblast, RoP + Combust + Trinkets, Phoenix (+FB if precast didnt crit), Pyro,...
You should always be using combustion at the end of casting rune, or the end of combust will lose that 50% bonus, which is especially important, because of pyretic incantation. Either that or at the end of rune you won't have combustion. Because of this, you can't cast combustion at the end of a spell that can crit. However, you also dont want to delay your cooldowns for too long, so that's why you use phoenix (and fb if necessary), so as few globals are wasted as possible. Two phoenixes and three fb should be enough for combustion, so using a phoenix to kickstart the chain makes it much more reliable, though that becomes much less of a sacrifice once you have reborn. Using a fb if the precast doesn't crit is also worth a more reliable opener, and if you really need to you can scorch at the end of the buff.
During the fight you do want to start with a heating up if possible, since you're not going to have three charges of phoenix. You can technically start with a hot streak if you dont fire off the pyro, and start casting rune instead of the fireball, but the time to cast rune and two more gcds for pyro isnt enough for that second charge to come back.