You soak it in a very dilute bath. User wrk101 knows a lot about oxalic acid because he's a scientist, yet he made me learn for myself. The dilution I used was pretty intense. I think I used like a quarter cup of the powder for every gallon of water, but I'm not sure if I remember. You want your dilution to be about 0.2% oxalic acid.

If you don't use bicarbonate of soda (baking soda) to rinse, then the oxalic acid won't be neutralized and will continue doing its work. You don't want this because if left, oxalic acid can actually be a catalyst for more rust to form (or some such). When I soaked my frame, I took it out of the bath, sprayed with water, then got a bucket with water and baking soda and starting dumping down the tubes of the frame. Then, I took some more baking soda and threw it in my bath since I was done soaking parts and wanted to neutralize the solution before dumping the liquid.