Drag the stickman's joints to new positions. When you are happy with a pose, click "Set Key" and the pose will be saved as an animation frame. Repeat until you have a decent number of frames, then play the sequence and watch your animation! New save function: Once you've made an animation you're happy with, you can click "Generate Save Code" and share your animation with other Scratchers! To load and play a save code, simply click "Load Save" and then "Play Sequence."
A number of similar projects exist on Scratch, most of them better than this one. Go look at them instead. Will add new backdrop options soon, as per @ShadowShells002 's request!