For runners, I used UHMW strips from Lee Valley. They're cut to fit exactly into the standard-sized mitre slots. They slide wells and don't bulge when you put a screw through them. I used hardwood as runners on my first sled and had to deal with change in temperature and humidity (I don't have a shop. I work outdoors.) Rockler or Woodcraft should carry UHMW runners too.

Most of your time will be spent adjusting the near fence so it's square to the cut. Trial and error, and a lot of patience. Goes without saying that before building the sled, blade has to be 100% parallel to the mitre slots.

Once you get used to using a sled, you'll want to build a family of them. One for mitre cuts. One for 45 degree bevels. One for dado blades. etc.