If you have an overflow box you will automatically have a surface skimmer.
It's not hard to build a sump, you just have to decide what you want out of it. If you want to put a skimmer in there you'll have to measure and build the baffles accordingly. You want the skimmer compartment to always have a constant water level.
If you want to do a refugium you'll build that similar to a skimmer compartment where your water level is always constant.
You also may want to put some space for a bit of particle filtration or to put bioballs or something, depending on what kind of sw setup you're keeping.
There's not much room to work with in a 10g but if you're creative and you know what you want you should be able to make a pretty good sump.
This is full of really good information about sumps:
http://www.reefkeeping.com/issues/2003-01/gt/index.php
The main thing, regardless of what style you go with, is you'll need to leave enough room to handle the water volume coming out of your main tank if your power shuts off. Other than that there's not much to worry about.
The only worry is your overflow box not repriming assuming your tank is not drilled, and it's not a huge risk as long as you get/build a good overflow box and test it repeatedly.
