Add a new trigger, then you need to add conditions and effects in that trigger :)

I'm not quite sure what your asking, but I'll try and walk you through it step by step.

first go to map editor and create a new senario.

Do what you want, make the map etc. If you want to modify a unit (without a whole lot of fuss) make sure that they are already on the field, and positioned where you want it.)

Then go to the 'Triggers' tab, and click it. A bar should pop up on the Left side with 2 fields. The first says "Scenario triggers" and the second says "Conditions and Effects"

Click, 'new' under the triggers tab. This should pop up a new trigger. Trigger 0. Each individualized 'formula' for lack of a better word is contained within a separate trigger.

For what you want, base effect changes, you can probably fit them all under one trigger with no problem. More complicated scenarios, involving multiple if then situations will require more triggers.

Think of conditions and effects like If then equations. IF this specific condition is fulfilled, THEN that specific effect takes place. If you want the stat changes to take place at the very start of the scenario then simply leave the conditions blank.

Back to the walk through. Click New effect. Scroll down to Change object name. Type in your desired name. Click your unit (the one you want changed), click set object. I usually spam this because AOE triggers are really bad about this. move your screen slightly and click go to object. If it worked the screen should realign with your newly modified character. If it doesn't keep clicking him. and pressing set object till it does.

Do the same thing with Effect, Change Object HP and Effect, Change Object Attack.

After that make sure to test and make sure everything works. and "SAVE BEFORE YOU TEST", especially if you have wonky triggers. I can't count the number of times I've crashed my game and had to start over on my triggers.

Good luck.

^Nice explanation!