First of all stop yourself there you aren't going to snap your fingers and get 91 rune crafting overnight. Rune crafting is a slow and tedious skill which if you don't go into it with the right frame of mind it will chew you up and spit you out. Still think you're up for it?
Well train on whatever you would like till you can craft fire runes. Crafting fire runes I'd recommend using a ring of duelling teleport to duel arena craft your runes teleport to castle wars and bank rinse and repeat. Once you hit 44 rune crafting you can now craft nats. You have multiple options now but the one I'd recommend is using the abyss to craft nats till 91.
If I was you though unless you can dedicate months into rune crafting and have the patience to do it just set yourself smaller goals and see how you get along. I'd be very surprised if you can stay motivated and stick to 1-91 rune crafting.