I did a cheap paint job once. I'm mechanically inclined, so for me it was not a problem to disassemble the bike. Used 1000 grit sand paper to remove the gloss (opening pore) from the paint and then washed the frame with kitchen soap. Let air dry without touching it too much.
With clean hands, tape any piece I didn't wanted covered, because I didn't completely undo the bike. At this point, One complete can of spray paint was enough. I finished the job with stickers that I had printed myself at home.
Sanding is laborious, but completely necessary. Opening pore in the paint creates a better surface for the new paint to adhere to. Washing is crucial to remove grease and dust, both of them prevent paint adhesion. For a strict budget project it is better to conserve original paint because it is already well adhered to the frame and it works gerat as a base coat. It is easier for new paint to adhere to porous, clean paint than to bare metal. Do not sand too much, since factory paint is really thin, and some times you wont find a different color base to guide. Use a flashlight (torch) to check if you removed gloss and that's enough.
Masking any part that should not be painted is highly advisable. For a start, paint should not get into any pivot, bearing, axle. But second, correctly masking the bike before painting gives the final work a better look (it can be so good it doesn't look cheap nor even home made). I recommend removing wheels, cables, derailleurs, handlebar, stem and crank set. Bottom bracket can be masked. Also mask fork bearings (headset) and seatpost. Leaving the seatpost is advisable to have a "handle" so you can move the frame while painting so you don't damage the paint job.)
The trick to paint a bike (or anything) is to use several light coats instead of a single thick coat. Start moving the can before you press the nozzle and use quick, consistent movements, continue moving the can after you release the nozzle. If you watch any car restoration shows or similar, you'd see that they do this with their paint guns.
The first coat must be so light that the original color easily shows through. You should be able to give the frame and fork 3 coats from a single paint can. Let dry each coat before applying the next. The can label should tell the drying time.
It is absolutely necessary to let completely dry the paint before reassembling the bike because the paint may be "touch dry" but still not completely set. If you handle the frame at this time you will smear the paint or cause finger print marks on it.
Finally, some stickers can help to make the bike look better. In my case I used inkjet printer on plain paper, then I used clean vinyl paper to cover and adhere it to the bike. How ever It may be cheap enough to have them professionally printed. At this time (2014) in my country they charge around 1 euro to print a sticker the size of a letter size (A4) sheet of paper.
For reference, in my country (Honduras) a can of paint can cost the equivalent from 2 to 4 Euros, a sheet of sandpaper costs cents, and a roll of masking tape is like half an Euro. So the hole project is around 5 to 7 euro, plus your workmanship.
I really advise you on being patient and not paint several parts with the same paint, because that usually gives a bad look, and you may end up liking the bike even less than you like it now.