Look at the yeast articles section of MrMalty.com. Jamil has written an excellent article on how to make a yeast starter, including calculating the right amounts of water and DME. Do yourself a favor and buy a stirplate and 2000 ml Erlenmeyer flask (maybe for next time). It is definitely a worthwhile investment, because it speeds up making yeast starters to a workable timeframe, and improves the chances of a ending up with a problem free starter, with a substantially larger population of healthy yeast than can be had with a non-stirplate starter.