When you put the servo vertically into the frame the two ridges on both sides of the servo snap into corresponding frame slots and the servos is locked in place. If you want to get the servo out of the frame use a knife or screwdriver for going in-between frame and servo. Widen the frame a little bit and the servo pops out. (Those ridges can be sanded off if you would prefer not to use the snap in system, that would allow the servo to be slid and out, and retained with either the following screw method or by taping over the servo and frame.) 
 
 

In addition to the snap in parts you have two half-round shapes on top-sides of the servocase and their counterparts within the frame. When the servo has been put into the frame both parts build a hole in which to screw in the 2 mm screws supplied with the servo for securing the servo. (This system can be used with or without the snap-in feature.)  

When the frame made of glass reinforced material has been sanded and will be fixed with epoxy in a sheeted foam wing we recommend to instal the supplied plywood strips on both sides of the frame (see photo). 
 
   
 

That way you reinforce the recess, adding substantial strength. If the plywood strips have been installed on both sides of the frame it is hard to widen the frame. In that case and if mounting in a hollow wing you may not want to use the snap in feature. Then, as mentioned above, just cut off or sand off the two ridges on both sides of the case. 
 
 

Having removed these ridges the servo easily slips into the frame and can to be secured with the two 2 mm-screws supplied (or taped). Two screw holes are created once the servo is seated in the mounting frame. In addition, there is a ridge on the bottom of the frame that seats into a recess on the servo's bottom case, holding the servo at the proper position in the frame. 

The vacuum formed cover is cut into two pieces, producing two aerodynamically shaped pieces that cover and protect the servo, servoarm and pushrod. Fix the cover with transparent tape along its outer edges - doublesided tape is not recommended as it can be extremely difficult to get the cover off, and can cause you to damage your wings skin material.