Step 1: Adding the Center
To start building your mall all you have to do is build a rectangle either 7x5 or 5x3 or even 9x7 path units. I prefer to use 7x5 though.
Step 2: How to Add a Food Court
If you wish to add a food court as your coutyard,
1. first you must delete the outer peremeter of the center area
2. next you have to make the center into a checkerboard pattern
3. then you have to add benches all around each path peice
4. then you have to fill the "checkerboard" in
5. finnally you have to add the outer perimeter on again (sorry, I forgot to put a pic for this step)
If you wish to have a fountain for the courtyard, skip this step.
Step 3: Adding a Fountain
If you wish to have a fountain as your courtyard, (I prefer to use a fountain instead of a food court.)
1. delete the inner area of the center path thing
2. dig a hole in the ground in the area that was deleted
3. fill the hole with water
4. (optional) add a type of fountain in the center of the pool (shown in next step)
Step 4: Adding Floors
And next to add the stairs.
1. add stairs in the way shown
2. add path that forms a perimeter directly over the floor's perimeter below
3. (optional) add a third floor (you can have a maximum of only 3 floors (unless your building underground) because the game does not allow you to place the stalls any higher)
If you want to only have a single floor in your mall, then skip this step and add your stalls & walls. I will make another instructable on how to build a single floor/ underground RCT3 mall.
Step 5: Adding Stalls
First you just have to build a bathroom on each floor on one side of the stairs and then just build one of the other three neccesities(ATM, First Aid, Information.) on the other side of the stairs
Finnally comes the time where you get to add the stalls.
The stalls that don't fit can go on the top floor. (you'll see what I mean once you start adding the stalls.) If the stalls won't fill the entire mall, you can just build more than one of some stalls.
Step 6: Adding Walls
Now all you have left to do is build the walls. Oh and I almost forgot...
1. make a path underneath the stairs that is 3x3 path units, you can also make the mall floor concrete, 'cause you don't see grass growing inside a mall that often
2. add walls around the shops and the stairs/ path; for 1 floor high make it 3 full hieght walls high; for 2 floors, 5; and for 3 floors, 7.
3. after that, add an outer wall that is spaced from the inner wall by 1 unit, except for on the front side. the mall should now be 11x10. Next you have to put the big window at the front of the mall like so. Finally it is time for the roof, so just add the roof on top. you can also add a skylight directly over the pool/ food court if you wish.
Custumise your mall all you want, from a balcony to a hundred stories to an indoor roller coaster. You can try to apply a similar concept when building huge indoor parks like the one i'm working on: http://www.youtube.com/watch?v=Jga0_FqVqpw