I wanted to build on the information from ratchet freak's answer
Concerning the runners and gauge slots of the table saw
Runners in most cases are sized to the tables mitre gauge slots. A dimentionally stable material is recommended (not required) so that the runners will not change in size too much and risk the sled binding. People still use wood obviously and the tip there would be to make sure the grain is parallel to the gauge slot.
Regardless of the runners you use you should always test the sled before running it. Not necessarily everyday but check often. Clear the gauge slots of debris and run your sled back and forth with the saw off to be sure that the movement is fluid.
The sled own safety features
The rear fence is most important as it gives a true surface for the wood to rest against. It reduces the chance of kickback to next to none. All cuts should be made with the wood resting against the fence. Ensure that your stops are also secured as you are going to be putting some pressure on them. You do not move, nor want to move, the work while the sled is in motion.
Another great thing about the sled is that it functions as a zero clearance insert. In order to use that feature properly the sled is usually partnered with the blade it was designed with. You basic blades are 1/8th of an inch in width so this might not be a big deal. Note that not all blades are put to the same standards and that there could be a minor variance. This comes back to testing before running.
Additional features for the sled
Again, ratchet freak's answer covers this area but I wanted to at least add pictures.
Image from lumberjocks
You can see an acrylic/plexglass guard that runs above the blade and a box guard on the end to help when you extend the sled farther.
Not all sleds have either of these features but you can never be too safe and these cause virtually no operational issues as long as they are secure.
Your hands and other objects
Keep them on the fence and if possible use clamps to keep objects in the sled secure during operation. Become a clairvoyant when you are about to make a pass with the sled. Know where your hands, clamps and work piece are going to be. This can help ensure that no object will come close to the blade path.
When it comes to clamps there are a slew of optional items you can use like t-tracks that can help with frequent stop positional changes.
Like covered in this answer don't have the blade higher than it needs to be. Also make sure that the blade height wont cut the fences while making a operational pass. Again this refers to a point I made earlier.