I just built this workstation I was fortunate to have all the materials except the table top & casters. Couple things I changed. Not wanting to waste wood I took four 2x4x8 stud lumber and lined them up against a stop screwed in the outside members to keep them snug then measure 5’-6” and made one cut thru all four to create the box frame. Did the same with the 2x6 legs lined them up and made one cut through all 4 . I installed the the table top last screwed into place 2 sides then using a fence cut the other 2 sides in place.
I also cut the table top to have a 2 1/2” overlap all the way around this offers more clamping areas. For the bottom table I had some 1x6 lumber that I used.
I installed the legs by placing the top & bottom box frames on their sides long axis and screwing in the legs screw side out then flipping.