When I first fitted these motors, the question would be quite how? Like most motors for drones, these are 'outrunners' with a canister that itself rotates in place of a rotor.
Accordingly they include a static base with an array of bolt-holes, which in turn means that there has to be access from beneath for a screwdriver. Some motors include an adapter which pitches the motor-mounts beyond the diameter of the canister, where they can be accessed from either above or below.
As to being set the task originally, I used a simple transfer technique and painted the underside of the motor before applying it to a paper template ~ the way that as kids we made potato prints.
Manufacturers like Boeing need neither potatoes nor poster paint to attach engines, but what they have instead is CAD/CAM. In the absence of this I suggest you get into a word-processing program and draft a template to print off and apply to the brackets at each corner of the airframe.
Mark with a centre-punch and break for tea prior to drilling, which needs be accurate.
