Download the Crazyflie frame v1.7 files from http://www.thingiverse.com/thing:90566.
1 of Crazyflie frame v1.7 (frame in).stl
1 of Crazyflie frame v1.7 (frame out).stl
4 of Crazyflie frame v1.7 (motor mount).stl
2 of Crazyflie frame v1.7 (junctions).stl
Insert the motor mounts in the motor base as shown below. Note that the motor mount part has a channel that should allow for the motor cables to pass through, be especially carefull because a clipped motor cable means an unhappy Crazyflie.
Fit all four mounts into the motors, these should fit tightly into the Crazyflie plastic motor supports you can use some double sided adhesive tape to help adhesion, I have not used any adhesive im my case.
Pass the flie between the Crazyflie frame v1.7 (frame in) part, align the motor mounts bottom notches with the inner frame part and press down until the frame snaps in place on the motor mount notches.
The frame should snap in place into the motor mount notches as depicted below.
The assembly should look like depicted below. Make sure the propeller stays clear of the top of the frame by carefully pressing the motors into the mounts.
Pass the Crazyflie frame v1.7 (frame out) part on the outside of the current assembly and align with the motor mounts bottom notches. Press down onto the free motor mounts until the frame snaps in place into the mount bottom notches.
You should now have something that looks like a frame. All you need to do is to add the 2 Crazyflie frame v1.7 (junctions) parts to the top an bottom frame intersections so as to add strength and keep the frame square.
You can glue the junctions and motor mounts to the frame using some fast contact glue or some acetone in case your parts are printed in ABS.
By the way I have used a diferent colour for one of the motor mounts because it helps as a reference orientation when in flying.