Step 1: Tip 1: Milling the Sled Runners
Tip #1: Since most wood movement is with the grain, mill the sled runners with the grain running vertically as shown in the photo. This will minimize or eliminate side to side expansion of the runner in the miter slot.
Miter slots can vary slightly in size but most are 3/4" wide x 3/8" deep. Most plans call for milling the runners for a snug yet smooth sliding fit with no side to side play (also called slop). However, if the runner width swells too much it can become a problem and, as I mentioned, this is where I ran into trouble with my first sled. The good news: Tip number two allows some wiggle room when milling the runners to width.
Note: The height of the runners should be less than 3/8" so there will be some space under the runner for sawdust debris