It would be a good idea to lay out your legs first. A dry run will minimizes mistakes later.

Start with the two longest leg pieces. Notice you will need to build two leg sets that are mirrored. Attach the 17 1/2" long leg pieces to the bottoms with 1 1/4" finish nails and wood glue. Take great care to line edges up - you want to create the illusion of a solid wood leg here. This will save you sanding work later!

Do the same for the shorter full legs. Note that the middle angles are all 20 degrees off square, but top and bottom angles are 35 degrees off square.

Position a leg set including one short and one long leg, crossing in the middle. Check to make sure the bottom overall spread is 23 1/8" exactly. Glue and nail down when satisfied. Build remaining leg in mirror.

Fit the long top leg with the remaining short filler piece. Nail down.

Position the side aprons in this notch and glue and nail down.

Notice the side aprons both sit to the inside. Attach back apron as directed in diagram.

For a drawerless desk, simply add the front apron too.

Bottom stretcher is a 1x3. Attach same as back apron.

Followed by the tabletop. If you have a pocket hole jig, you will want to attach through aprons with 3/4" pocket holes and 1 1/4" pocket hole screws (drill pocket holes PRIOR to step 7). Otherwise, nails and glue down with 2" finish nails.

For the drawer, build as shown above. Attach plywood to the bottom with 1 1/4" finish nails and wood glue. Check to make sure your drawer is square and overall 1" less in width than the opening in the desk.

Attach drawer members to drawer bottom side corners. Attach cabinet members to insides of desk.  Follow the instructions in this plan for installing drawers.