This is how I have mine mounted on my Senior Falcon (which is supposed to use a center "bellcrank" servo, but the bellcrank system just doesn't work right in this plane). I outlined and cut 2 "slots" out of the wings in line with the control horns (and used the existing control rod exit point as a reference). Then I chopped out the internal foam core and stuck the servos in. I later added 2 small peices of wood glued to the wing to screw the servos into for extra support.

I don't know how well it's going to do as I haven't gotten it airborne yet.... my new tank should be here this week and ALL the peices will be in place

..... but Mac's right, what do the instructions say? I've seen wing servos that were mounted sideways inside the wing and the wing panel was bolted on top of it with JUST the servo horn sticking through and I've also seen them bolted on sideways. Which way is the "right" way is probably dependant on what kind of plane you're flying. Mine is a 70" wingspan "trainer" style, so a little extra near the fuselage on each side isn't going to matter much. On the other hand, if this was a sport or aerobatic plane, it might make a HUGE difference in handling....

I'd follow the instructions. If you can't find anything in the instructions, contact the manufacturer (find them on the web, there's always a "tech support" email or phone number). They'll point you in the right direction and they're USUALLY happy to help. ( I even got ORIGINAL decals for my Falcon for FREE from Goldberg just because they were probably SHOCKED to see one still flying after 30 years!)