There are many ways you can choose to mount your motor-wheel assemblies to the platform.
The important thing is to mount them in parallel so that the two wheels are pointed in the same direction. Also, they should be the same distance forward and aft (specifically, both right in the middle). And the tires should have plenty of clearance in the wheel cutout holes in the platform.
Usually, the motors are mounted on the bottom side of the platform. This will give you a platform height of about 2 1/4 inches. If, for some reason, you want the platform lower, you can mount the motors to the top surface which will give a platform about 1 1/4 inches high.
A number of different mounting techniques are shown below. You can make your own out of aluminum or brass bar or angle. You can buy a servo mounting box or mounting brackets from a hobby store. You can mount the servo under a retaining plate. This has the advantage of making the servo angle and position easy to change. Or, if you're really lazy, like me, you can use foam double sided tape to hold the motors in place. But, make sure you get them on straight. The position is difficult to change.
Brackets made from flat brass stock (available at hobby stores)
Or make similar brackets from aluminum angle (your local Home Depot)
Servo mounting box. Also at hobby stores.
A mounting plate
Mounting brackets. More hobby store stuff.