1. Saddle-to-Stem Distance
Take your elbow and lean it against the front of your saddle. The distance between saddle and stem is correct if your middle finger is touching the middle of your steer tube.
2. Saddle Height
Place your armpit on the saddle of your bike and hang your arm down towards the pedal. The saddle height is correct when your middle finger touches the top of the bottom bracket.
3. Stem Height & Length
Anchor your thumb to the middle of the steer tube and place your fingers on the stem on your handlebars. Your middle finger should reach the end of the stem. Keep in mind that stem length isn’t actually adjustable, so you should check this carefully when buying your bike.