Just make sure to place the battery under the turtle deck and not under the canopy. Otherwise you will have trouble getting the CofG right with a 3s 2200 mAh battery (which is the minimum capacity in my opinion - giving you about 6 to 8 minutes flighttime).
I recommend that you assemble the plane as shown in the doc-file instructions postet above, but don't cut out the hatch (skip step 10 and 11). Instead you cut out the turtle deck after the build is finished and make it become a service door using magnets and tape. You can safely cut away the vertical, triangle shaped part of foam under the turtle deck (I strongly recommend you dont put any glue there during assembly). You might also want to build a little box for the battery and cut even deeper into the foam of the fuselage under the turtle deck to make the foam-battery-box recess into it. This will put the battery almost right on the roll axis, which will make the plane behave more smooth.
Here are two pictures from two of my F-22 V 2 that show the placement of the battery under the turtle deck in a foam box (the pictures show two different planes):
Gravity can be cruel !