In his book, Seed Germination Theory and Practice, Norman Deno describes this very easy method for using GA3.
A 2.5" square of paper towel is folded 3 times to give a pad .5" x 1". This is moistened with water and the seeds are placed on top. Over the seeds is sprinkled the amount of GA3 that can be balanced on the narrow end of a flat toothpick. The pad is then placed in a plastic baggie. After 24 hours the seeds can be sown in the normal fashion. This technique gives a concentration of 1000 ppm.
For a fuller explanation of GA3, see Gardenfundamentals by Robert Pavlis, an ORG&HPS member.