Wipe down the blades of your scissors with rubbing alcohol. Tools can spread disease throughout trees.
Choose one or two branches on your apple tree to begin thinning blossoms and dive in. Without many helpers, you can't thin an entire tree in one day. Work on the tree over the course of several days.
Identify the biggest, healthiest looking blossom in each cluster. Leave that one and remove the other blossoms around it in each cluster of blossoms, snipping them with scissors or plucking them with your fingers.
Thin one group at a time, taking care not to damage the apple blossoms that you intend to leave.
Use a ladder, as needed, to reach blossom clusters.