Place them on a hard and thick surface like a table or stairs. Most stacks of magnets with a diameter up to 30mm thickness can be slid apart by hand. Just grip or slide your fingernails between the magnets at the point where you want to separate them and slide them off the stack.
Be careful to move the magnet(s) far enough away so that they don't jump back together and pinch your fingers.
For magnets with a diameter larger than 30mm thickness (the thickness determines the strength of the magnet), you may not be able to slide them apart, and it may be worth investing in a simple separator made of non-magnetic material such as wood or stainless steel. However, it's worth improvising by using the edge of a table, for example, as a fulcrum point to separate larger magnets, but again, be careful to separate them quickly and move them apart to prevent them from rejoining unexpectedly. The consequences of such uncontrolled joining can be not only painful if you pinch your fingers or skin, but also, due to the tremendous force of the magnets, they can crack and shards can be shot in many directions, so eye protection should be used.
Remember!
Gloves will help protect your hands from being pinched by neodymium magnets, and glasses from "shots" from uncontrollably broken magnets.