The instructions are pretty good, there's just a couple tips I would give to make it smoother for you.
--Cut off the extra bit of axle with the wheels on and fasteners attached to avoid cutting off too much.
--Try and have the main gear legs as short as possible, as in wheels as close to retract unit as possible. Make the nose gear leg long enough that you have to cut a bit from the stock plastic surround piece to make the wheel fit. This helps give the plane a more "up" attitude to make take-off easier because she is heavier and needs a longer take-off run with the new gear on.