Add all your sprites and characters To add an extra scene, change the sprite with the "Back at the end" message block change it to "next stage",add a new stage, And code it (when you have done it, add the "Back at the end" message block to the end)
For the least lag click here: https://forkphorus.github.io/#554137800 (some of the sounds may be wrong on there(to avoid that, use turbowarp))
