I would also recommend you go to a salon, as someone who's gone black > blonde. Sorry, I know you said you can't afford it but I honestly think it'd be worth saving up and getting it done properly. I had to have my hair bleached 3 times; sat there for about 3 hours and it might be worth noting that the hairdresser put on those big steaming hoods instead of just leaving me to sit while the bleach worked. And yes, the hair does go brown after the first dye (a rather pretty golden-brown). I took photos between each bleaching because I wanted to show people but I'm not sure how to upload them. :'-D
Anyways, if you do it wrong (i.e. get too much on your scalp), especially with strong bleach, you could end up with some nasty chemical burns. If you don't apply closely enough to the roots you'd end up with a funky pudding top instead. In either case, you'd likely have to pay extra to get it fixed. Just not worth it.
Last notes: you don't have to get any special hair treatments done at the salon, but it's worth asking if they could use conditioner (during the final wash at least). + I'm very fond of Redken's All-Soft Heavy Cream hair masque.