Once all is sanded, I select the two end supports
I always put the first seat slat in position and slide the two end pieces slightly left or right to insure the 'over hang' at each end is equal before I tighten the bench vice.
The seat has 4 of the slats and the other two are the back slats. Place all 4 seat slats in place and adjust until they have equal space between each. Remove the two in the center, then pick the back slat up so you can add glue to the horizontal edge of the support where it was resting. Gently replace that slat, check the overhang, and hold it there for a few seconds. Next do the same for the front slat.
Set the assembly aside, add glue to the center support, and place the support in the middle of the assembly area.
Thanks for looking....