Build a fire large enough to cover the rasp in coals. Leave overnight, come back in the morning and you are the proud new owner of an annealed rasp.
But, without having a forge you wont be able to retemper the knife, leaving it soft. It will still function as a knife, but it wont be as durable, or as sharp as youd probably like it to be. I suggest carefully working it without annealing. Dunk it in a bucket of water when it starts to get uncomfortable to hold.
Good luck. Dont forget some pictures when you get started.
Kid