First things first: Mark what sides go out! It would be a shame to put all that work into the legs and have them come out mismatched or have a dado on the wrong side.

Since my tablesaw doesn't support a 1.5" dado stack, I opted to make multiple passes of a 3/8" stack. I made each operation in pairs, cutting both legs at each fence setting before moving on.

I found the best method to make the cuts was:

1. Make 3 passes so it was almost to fit.
2. Set the shelf in the groove and mark the top with a knife
3. Sneak up on that line and test fit the shelf

This worked out for the most part for me. We'll revisit where it didn't in a later section.

Because I wanted a solid joint to glue up, I opted to clean the bottoms of each dado with a router plane. This ensures that each dado is exactly the same depth and no irregularities from the saw remain.

Here you see where we start, rough bottom off the saw.

Take a wide chisel and sever the fibers on the edge of the joint. This makes it easy to make the cross grain shavings with the router.

After the fibers have been cut, set your router to a mild cut and come in from both sides of the dado. Don't try and make the cut from all one direction. This will blow out on the opposite side and make you sad. By cutting in from both sides you keep a crisp entry point. 

If you find it's really hard to make the cut, back off on the depth and try again. More often than not, you just need to be a little less aggressive with your cut.

You can see what kind of a difference this makes in your dados compared to those right off the saw. Now you'll have a smooth surface for good glue adhesion.