Here's a general routing video. Not bad. The illustration is of a simple jig for cuttng dados or grooves. Clamp it in place so the fixed end runs right on the line for the top of the groove or dado, place a piece the same thickness as what you want to glue into the dado/groove. I always add a thick piece of paper, an old glossy political mailing will do. Then bring the bottom movable part up against the edge of the piece you're going to glue into the dado/groove. This will deliver a well fitted exact width opening.
The bit for this should be smaller in diameter than the width of the dado. It should have a top mounted bearing as shown. I prefer to use a fairly short bit and make several passes. The bit must produce a nice flat bottom. You will make one pass with it with the bearing up against the top guide, then pull it down and across the bottom guide. Simple, precise. Make 3 passes, no more than 1/8th at per pass. The bearing must remain against the guide so I'd make the guide of at least half inch stock. The illustrated jig requires that the top be attached to the side rails at an exact 90 degree angle. Or, if it's off slightly, oversized to you can clamp the top piece to the work piece.
Finally, since you're new to this woodworking habit, I've attached a fairly long PDF of the 17 plus things that accelerated my learning curve. It may help you avoid some costly missteps. It has lots of pictures.
Here's the video:
Here's a drawing of the jig, and a picture of a mortising bit you use for fairly shallow flat bottom dados. I disagree on the bit sets. But if you get a set, make sure it has the bit you need for cutting flat bottom dados. Freud makes great general purpose bits.