After cutting centered grooves in each of the rails and stiles, the next step is to cut the tenons in the ends of the two rails. These two tenons will fit into the grooves on the stiles, which at that point will act as a mortise and tenon joint. These joints will determine the strength of the door, so the tenons must fit snugly into the mortises on the stiles.
With your stacked dado blade still in the table saw set to 1/4" width, adjust the height of the blade to about 3/16" above the table. Then, clamp a sacrificial board against the leading half of your fence (but at least a few inches away from the blade). Since you'll be cutting the tenons using your miter gauge, this board will act only as a measuring guide. See the image above to view a proper set-up.
Adjust your fence so that the measuring guide board on the fence is 5/16" away from the opposite edge of the dado blade. Then, lay one of the rails flat on the table with the end against the measuring board. Using the miter gauge set to 90-degrees, make the first tenon cut. After clearing the blade, lift the board off the table, return the miter gauge to the starting position and put the board back in place, this time to cut the remaining portion of the tenon (the board should not be touching the measuring board this time). Finish the tenon by making a this second cut before flipping the board over and cutting the other half of this tenon.
Attempt to dry-fit the tenon into the mortise on one of the stiles. The tenon should be too thick to fit into the mortise (which is by design). This will allow you to make gradual, appropriate height adjustments and re-cutting of the tenons until they fit snugly into the mortises of the stiles.
Cut all four tenons in the ends of the rails using the same steps.