Excellent idea to build shop cabinets to build up your skill and confidence.
I second using pocket hole jigs to make face frames. The important part is that you make certain your saw is set up EXACTLY 90 degrees to the table so your cuts are square. If they are square, you can measure the cabinet line two pieces of your faceframe stock on top, then measure the remaining width to get an exact fit. If you want it flush, cut the exact length, if you want 1/8th overhang, add a quarter to your measure for the rails.
Drill the pocket holes on the ends of the rails using the jig in the Kreg kit. When you screw in the pocket hole the joint will tighten up and be fairly strong. If you were accurate in cutting the 90s, it will also be square. I prefer the screws with the square head, they drive in more reliably than the phillips type. Use fine thread for hard wood, coarse for soft. I find 1 1/4 length is perfect for pocket holes. The pocket holes go on the edge of the cabinet so you don't see them.
All this assumes your cabinets are square. That is largely a function of your cuts, which means your table saw's blade is perfect 90 to the table. I popped for $30 and got a Wixey Digital Angle gauge from Amazon. It will get you to 90 exactly every time.
Square cabinets are also aided by use of proper clamping. There are a number of ways to do this.
Parallel clamps have wide jaws that are engineered to hold cabinets square during glue up. Augment this with a 90 degree corner brace clamped to the cabinet sides to hold everything square. Bessy makes the most common parallel clamps, but I got the Jet models instead because they have a quick release lever. They are expensive and you need to get one a foot longer than the maximum height of the cabinets you're making.
Rockler makes a plastic, L shaped gadget you can clamp into the corners.
You can use pocket screws to put together the carcuses. If they're cut square, these will pull the cabinet square. You'll use glue as well, so you can fit dry, open the joint by unscrewing the pocket hole screw, glue, then retighten. Kreg makes a clamp that helps with this process.
Pocket holes are not pretty. Not a problem on the top of a wall cabinet, but not so nice underneath. Kreg makes plugs you can cover and fill the opening with, but it will take patience and a japanese saw to do this. It requires using a wood filler, so finishing can be problematic. My kitchen wall cabinets are not finished underneath because we installed LED lights.
For base cabinets, you can hide the pocket holes with a counter top, and put them underneath the bottom. BTW, buy screws 500 at a time, they go fast on a cabinet project.
This is a lot of detail, but the real point is make sure your blade(s) are set
exactly to 90 degrees.