I keep a bag of dry malt extract in my cupboard, just for making starters. You don't need hops or anything else- just boil 1/2 cup DME in 2 cups of water and then cool it. I put it in a growler with sanitized foil over the top and shake it up well and add the yeast. You can "step it up" the next day by adding some fresh wort that you make the same way and cool. If I'm doing a regular beer, I don't add fresh wort the next day- I just use it as is. For a lager, or for a higher OG beer, I step it up.
I like to make my lager starters a week or so in advance, so I can chill the container, and then decant the spent wort. For smaller starters, I just pitch the whole thing. Swirl well, to get all the yeast off of the bottom and into your wort.