I found myself needing a couple of sawhorses, and I have a bunch of pallets... I went online and found a guide, but it was (imho) somewhat of hard to follow with no pics. So I used the basic idea and built my own. Enjoy.

Step 1: Tools and Materials

This is my list of tools and materials it is by no means absolute but it is a start.
A crowbar, wonder bar or nail puller.
A claw hammer.
A tape measure.
A pencil.
A speed square.
A a skill saw.
A drill.
A drill bit, for pilot holes, the same size as your screws shaft.
A screw bit that fits your screws.
2 ½ or 3" and 1 ½ screws. Coated, galvanized, or not, your call.
A stack or so of pallets.
If you have better, different, or want to use store bought # 1 pine that's cool. I used pallets because they are free for me, and cheap for anybody. If you buy pallets be somewhat picky. Pallets get used hard, often the wood they are made out of is not of decent quality for anything but pallets. Pick good pallets.

Step 2: Cut List

Take apart the pallets with the wonder bar, crowbar, nail puller, etc, remove the nails, sort the lumber and, cut:

I use my speed square as a saw guide. Fast and easy.

From the 2x4s:
2 at 9" at 22% on both sides (7¾" on top and 9" on bottom).
2 at 12" as square as you can.
4 legs Your call here, 30 is standard,I cut mine for about a 36” sawhorse so my legs are 34” you could cut one of the ends at 22% for a fine fit. I didn't but it would have looked nice.

From the 1x4 slats:
2 4' inch wide, thickness is not critical, at your desired length. They are your sawhorses, you get to pick how wide they are. Try for good pieces as these will be the tops of your sawhorse (splinters hurt) 4' inch wide, thickness is not critical.
2 more at the same length for optionaly bracing of the legs.

Step 3:

Mark the center of the 9” piece on the side as well as on the bottom. Then, mark the center of the side of your 12" 2x4 on one end, and a line parallel to the cut 4 inches from the opposite end. This will be the T brace of your saw horse so get it centered and perpendicular accurately.

Step 4: Tee's

Drill 3 pilot holes from the bottom of the short angle ended 2x4 on its center line and then fit Extend your pilot holes into the 12' pieces. Screw together.

Step 5: Stand Tall and Be Done

You can cut the ends of your leg pieces off at 22% on the tops and bottoms for a clean look, I wanted to get done so i didn't.
Mark and drill pilot holes on the legs. Attach the legs to your Tee's. The top edge of each leg should be at the 4“ mark from earlier. secure at the top, and on the angled brace, with screws.
Install the 2 top pieces.
Brace 12” up the legs(optional but strong).

I did not finish mine, but you could if you have the time, and want to spend more on the finish materials than on the sawhorses.