Lumber premilled with tongue and groove edges saves a ton of time when creating large project parts, although most of the time it's used in paneling applications.
We've used tongue and groove cedar, for example, in several instances including a decorative fence gate and panels used in creating a planter. I'll focus on the planter application here.
Cut the parts to length. You can either cut them to the size you need, or cut them a little longer and trim after you assemble the panel.
Apply glue to the groove side and the tongue side.
Insert the tongue side into the groove side. You can try sliding it in from one end, which will push the glue out of the groove, or line it up and work it in from the side. If it gives you trouble, use a piece of scrap wood and a rubber mallet to tap it in place. Make sure it is fully seated, flush at one end, and wipe off the excess glue. Repeat for additional parts until you have the desired number of parts.
Allow the glue to dry. I prefer to place some weight on the assembly to keep the panel flat while the glue dries. You can clamp if you prefer. Because this panel is part of an assembly (essentially one side of a box), and will have a cleat attached to the inside face and a piece of trim attached to the top edge. I won't use any fasteners.
Cut the panel to width. Set up your table saw to trim off either the tongue or groove side first. I prefer the tongue side, mainly because in my experience the groove side will ride along a table saw fence a bit better. You want to make sure to fully cut off any taper that has been cut leading to the tongue so you have a square and flat edge. Cut off the groove side as your final width cut.
Cut the panel to length if needed. If you cut the parts longer than needed, now you can trim it as necessary to fit your desired dimension.