BLM/ THM feels a bit weird before you have fire 3 and blizzard 3. You'll see when you get there, but it sort of streamlines the whole process. For now though, on 3+ targets use fire 2 (unless your tank can't handle it, then just do one at a time) until you can't cast any more. Use transpose and cast either blizzard or blizzard 2 if there are enough mobs (I usually only bother with 4+) as well as thunder on a target that won't die in the next 18 seconds. When mp is full either transpose back to astral fire, or just start casting fire. I forget how the timing works out, but if you'd have to sit there with full mp waiting for transpose to come back up then you shouldn't -- just start casting fire/ fire 2.

Once you get astral fire 3/ umbral ice 3 you can skip transpose most of the time and just use it when combat ends and you're still in astral fire. This works because fire 3 will always give you 3 stacks of astral fire, regardless of what buffs you had before and blizzard 3 will always give you 3 stacks of umbral ice. The tier 3 version of both buffs also give greatly enhanced casting speed to the opposite element and umbral ice 3 refreshes your entire mp pool in 2 ticks (between 3.1 and 5.9 seconds essentially). It has a pretty significant effect on how BLM plays.