Try to figure out which side the loose screw is on. Put the cube on a flat surface with the bad side up. Remove the center cap on the top side. Unscrew the screw there. That side will dissassemble in a way that will be fairly easy to reassemble.
But if you aren't coordinated you can turn the thing into a pile of a couple hundred parts that are hard to reassemble.
To reassemble a 7x7x7 SS that is totally disassembled, note that it has an internal spider that has all the 3x3x3 components. These are assembled first, then the other parts. Assembling the 3x3x3 subset is the most difficult, especially if you've got lube all over it.
The 7x7x7 comes with a box that just fits it nicely. You can use this box to help reassemble it. I use the box to help make the 3x3x3 assembly and then use the box to assemble the other parts, one face at a time. By "one face at a time" I mean that I assemble the edge and face pieces that are visible through the top of the box. The last step is to tighten the screw. This makes the face more sturdy. Then I pull the cube out of the box, rotate it, reinsert it with another face showing.
The 3x3x3 pieces need to be put in so that the cube can be solved. So put on the edge caps and put the 3x3x3 pieces in such a way that you get a solved 3x3x3 cube. If you put the 3x3x3 pieces in randomly you'll get the same thing as with the usual 3x3x3. That is, you'll have a very high chance of getting a cube that needs to have an edge or corner twisted, or a pair of pieces swapped. With the usual 3x3x3 this isn't a big deal, but it is really hard to swap two 3x3x3 cubies on the 7x7x7 SS. Just put the remaining 192 pieces in any way you find them. The mathematical reason for this ability is that the remaining pieces have lots of duplicates. This means you can swap them around and still have a "solve". This prevents them from having "parity".
To do one of the faces, I first remove the screw completely and remove the center piece (but leave on the little loose shield that keeps the 3x3x3 pieces in place). The reason for doing this in the box is that the box keeps the 3x3x3 pieces stable while you reassemble the face. Then I put in the 24 edge pieces, two at a time. Then the 4 spindly little corner face pieces. Then the other 12 outer face pieces. Now you still need to put in the nine inner face pieces. There are various ways. I assemble them into a little 3x3 face and insert them all at the same time. Other people will do it different ways.
Taking apart your 7x7x7 SS is a lot of fun. If someone looks at the pile of 200+ parts they'll be immediately convinced that you're mentally deficient for taking it apart and no one will ever get it back together. (Uh, if you have trouble, look for help from someone who's mechanically inclined.) But actually it's fun, like assembling a jig-saw puzzle.
I take all my 7x7x7s apart regularly. They get nasty inside because of wear in the plastic and dust from the environment. I clean the parts with hair shampoo by shaking them all with a liter of water in a plastic bucket. I dump them out on a towel to dry, and then reassemble them. I love the feel of a freshly cleaned, relubed cube.
I like my cubes to be easy to turn but without a lot of play (so they're fast to turn but don't pop easily). So I've got "shortened springs" on my 7x7x7 SS. But if you make the springs short enough you find that the screws won't go in far enough to compress the springs! To get short springs to work you have to insert a little plastic piece that replaces some of the spring length. I make these from the insides of ball point pens. They stay in due to friction in the center pieces (but I remove them for cleaning). They go underneath the washers, so you can think of them as ultra-thick washers, made out of plastic cause it's cheap (free).
My best WCA solve on the 7x7x7 is pretty slow, 8:52.66, but it's the world record for ages 54 and above.
