Use soil that is well-drained with a high concentration of organic matter and you may not need to fertilize your basil at all.
Purchase some slow-acting granular fertilizer to spread around your basil plants at the beginning of the growing season if you're working with poor soil.
Spread the fertilizer very thin and try to get as close as you can to the root system of the basil plant. Follow the manufacturer's directions on the fertilizer as well.