This is a job you can do indoors on a rainy day. I've printed the spec for the motor that includes a technical drawing of the threaded holes on its base that are used to attach it. They require M3 bolts spaced around a 30mm diameter circle and for our purposes we need use only two (an advantage of square tubing being that motors sit flush on the arms and require no adapter).
Due holes in the re-used arms I've had to set the templates a half-inch nearer to the centre-body of the drone than I would have liked, but it is what it is. I've also used templates to locate motors on the underside of the arms at each end, and all will be centre-punched as a guide to drilling 3mm holes.
A further advantage of drilling through the depth of each arm is that the motors can be attached with bolts of a reasonable length, which is a whole lot easier to manage.
Separating the motors along the length of the arm in this way provides for easy access to fit motors from above or below: a reason I prefer the arrangement to co-axial ones that require special fixtures.
