did you freeze what you had? or is the frozen wort seperate.
Put a 1.030 to 1.040 solution (100grams in water that is filled to 1L) Boil solution and sanatize vessel for starter. People use Eurlymer flasks becasue they can do heat. I've sanatized a 3L wine bottle and a pitcher for drinks and hand no issues, but then I'm crazy that way. Put the the starter wort (1L to 1.5L is a good size )
Take your left over yeast and basically let it settle, then pour off the water. there should be a little water with the yeast enough to get it into solution and put into your starter. cover starter vessel with foil. Agitate (shake) the vessel, or use a stir plate for improved results.
A lot of people will tell you to 'rinse' it. This removes the less desirable solids. Alternativly you can just pitch what you have. the risk there is that with a starter you can atleast check for viablity. I don't remember the details of rinsing though (also called washing, but techincally only a rinse).
I think a rinse is this
Agitate trub yeast.
let site 1/2 hour. Pour out/decant solution (it hsould look cloudy). Let it sit again, pour out top water.
Basically you are time/gravity sorting for the 'upper solid' layer. The first to settle is the least desireable, and the top is water/liquid and not yeast.