Make sure you have cylinder on TDC and the valves held closed by compressed air. After compressing the spring, give the compressor a tap or two on top with a hammer to loosen the split collets or keepers that are positioned around the top of the valve stem. Block any oil drain holes in the head with paper towels etc as the keepers can get away. I keep a pencil magnet handy to retrieve them.
After you change the stem seal, place the spring and compressor back over the valve and push down on it as you replace the keepers. I use long tweezers and thick grease to hold the keepers. Once you are sure the keepers are correctly in place release the compressor and diesconnect the air line. I give the top of the valve a sharp rap with a hammer and punch to seat the keepers.