Guys, im trying to make a square on a cylinder surface with subsurface modifier but im having troubles with topology, anyone can help? Thx!
Select the edges of the hole, (call these original hole for ref)
extrude.
select the newly formed edges, inner loop and the corner ones, don’t select the original edges,
open you “N” panel, under transform, edge data, slide the crease to 1.
this should create hard edges around the inner hole, but allow the outer hole to retain the tube roundness.
in short, extrude, select new edges, crease 100%.
But i want to be square, not round 
it will be square, try it. the main bit is creasing the edges that run from the old hole to the new hole.
extrude->select edges you wanna make square->ctrl+b(bevel)->it will be sharp edged.
or
create new square when you are in edit mode, you can edit it seperately in edit mode, but in object mode it will be grouped to cylinder and move as one.
Thanks for the help guys!
you could also simply project a square hole onto the cylinder using the knife cut!
this will follow the cylinder shape
happy cl
Ok I don’t quite agree with the above solutions, using the crease modifier will create a fair amount of pinching. You have to remember that for a square to have sharp corners it needs supporting loops. The best solution to doing this is to simply increase the density of the initial mesh so that the hard edges can be sustained.
It’s a popular topic over at Polycount, please refer to this thread and Wiki page…
http://www.polycount.com/forum/showthread.php?t=56014
http://wiki.polycount.com/wiki/Subdivision_Surface_Modeling
Try starting with 32 and 48 sided cylinders to begin with.