I make small envelopes out of advertising flyers. I cut it into squares ranging from 5 to 8 or 9 inches, depending on how many seeds I will have. (Access to a paper cutter helps.) Fold the square in half diagonally. Fold down the centre tip on one side and then fold in the side points in thirds, tucking on inside the other. I generally secure it with a bit of tape at this point. Pour the seeds into the envelope and fold down the remaining flap. I secure it with tape. How much tape depends on how large the seeds are - more tape for tiny seeds that can slip out.
Later, once the seeds are really dry, I can put them into little ziplock plastic bags from the $ store. I collect a lot of hosta seeds.
Ann