Match it to your drilling template and true up both ends in the process. Now is the time I cut the board to length. You can use your jointer, taking very thin cuts, a drum sander, or just sandpaper adhered to a dead-flat surface such as your bench top or table-saw top. Once this is cured, re-insert the sliding door and flatten everything. Next, glue in the stop block – use glue on 3 surfaces – then clamp it as illustrated in this photo. Carefully slide the center piece in and use 2 additional C-clamps to pressurize the glued joints.