Make it drive and have big boy tomato so it flys faster than North Korea

@DownUnderWing make 2 engine sets one for crusing and one for takeoff

That helps the Short Take Off part but not always the Landing part. @MechWARRIOR57

make the plane go really fast with super engines

Ok thanks for the summary that helps a lot ;) @AstleyIndustries

Summary:
- keep your plane light
- if possible, use taildragger landing gear config
- use angle of attack and flaps
- use flat bottom airfoils

Ok hmmm thank you really helps I will ask if I have trouble @DownUnderWing

Flaps: I'm not entirely sure on this but I can speak from observations.
Flaps seem to work as an extra wing or control surface in simple planes. You can see this in action by placing the wings of an aircraft into rotators and adjusting them in flight. Your aircraft will either climb or descend depending on how you angle the wings. If the wings are in line with your CoG then the aircraft will rise. If it's in front or behind then the aircraft will pitch up or down depending on its location.
If you place flaps behind your wings then the same is true.
The aircraft will either pitch up or down if there is a distance between the surface and the CoG. If you can place the flaps as close to the CoG as possible then the net result is an increase in lift. This allows for earlier takeoffs and slower flight on landings due to the extra lift provided by the increased angle of attack of the flap surface. The larger the flap - the more lift produced.

Ok thanks a lot mate I will have a try @DownUnderWing

Ok thanks @Feanor

Ok thanks @RailfanEthan

Landing Gear Placement/Configuration: A lot of STOL aircraft are conventional gear setup (taildragger-two main wheels up front with small tail wheel at rear). The advantage to this setup is that the wings naturally sit with high angle of attack on takeoff and landing. The aircraft will produce lift as soon as you gain airspeed on takeoff without having to touch the elevators. On landing, the aircraft is landed in a nose high position which means that the airspeed will be low enough that the aircraft won't climb but produces high amount of lift until touchdown due to the angle of attack in its nose high landing attitude.
With nose gear the aircraft naturally sits in a level attitude so the angle of attack is lower and little lift is produced as you gain airspeed. You have to use the elevators in order to increase the angle of attack for takeoff. Simpleplanes doesn't simulate airflow from the propeller so you have to wait until your speed increases enough to allow the elevators to become effective in order to raise the nose and increase lift from the wings. The rear landing gear should be placed as close to and behind the CofG in order to allow the least amount of force from the elevators to rotate the aircraft into this nose high attitude for a short takeoff. It's a simple fulcrum so the closer the pivot point is to the CofG the easier it is to rotate and the sooner in your takeoff roll you can lift the nose to become airborne.

Low wing loading: the lower the wing loading - the slower you can fly whilst still maintaining lift. Two ways to have low wing loading - less weight and/or more wing surface area.

Control Surfaces: These need to be as effective as possible in order to maintain control at as slow speeds as possible. You need to be able to control the aircraft, particularly the nose right up until the stall point. Eg. If the aircraft stalls at 50mph then the controls need to be effective enough to keep the nose level all the way up to 50mph. They can however be too effective and cause the aircraft to stall above 50mph by causing the aircraft to exceed the capable AOA of the wings (these aircraft tend to snap one way or another when doing loops or violent pitches etc). This ties in with the relationship of the CoG and CoL. If you have small control surfaces and the CoL is very close to the CoG then your aircraft will be able to maintain its nose attitude for a lot longer than an aircraft with large control surfaces and a large distance between CoL and CoG.

CoL and CoG relationship: The CoL and CoG have a relationship in aircraft building. The CoL acts as a pivot point to rotate the CoG in the pitch axis. The close the CoL is to the CoG the less force is required to rotate it. A good STOL aircraft will have it's CoL and CoG close together as it allows for earlier rotation on takeoff and a more responsive aircraft on slow landings. As your aircraft gets slower then more force is naturally needed to control it as the control relies on airflow over the control surfaces. If your airflow reduces over the control surfaces then your surfaces will naturally produce less force - so if less force is available then we need a system where less force is more effective ie. a closer CoL to the CoG. As you adjust this ratio you will need to adjust the force applied from the control surfaces - the control surface size.

One trick that I do with STOLS is that I replace landing gears with skis made of fuselages. That way, the craft decelerates extremely fast, and don't worry: if the skis are made right, they will be just as sturdy as landing gears.

As long as you can keep the plane light, and place your col and com right, and make sure your wings are big enough you should have no troubles making a stol

You need low wing loading. This can be done by stacking wings on top of one another.

Usually they have two front wheels and one rear, for easier landing. And they're slowish. If you can use flaps, then do so (idk how).

What's STOL?

@DownUnderWing ? Rule of thumb? ETC