You would first need to:
1.- Setup movement and gravity (both are implied)
2.- Make a simple test background
3.- Create special statements based on the background you have (like not falling when you're on a platform).
That's pretty much the basics of the basics, but how to do it is up to you. The movement and gravity is very simple, but the rest depends on how advanced you want to get.
By increasing the Y position of the player by using a constant:
GRAVITY=6
INC PY,GRAVITY
SPOFS 0,PX,PY
And the jumps are performed when you set a opposing force (with negative value) that will nullify gravity and will gradually decrease:
VEC=-16
INC PY,(VEC+GRAVITY)
DEC VEC
SPOFS 0,PX,PY
