Check out my other various ergonomic Instructables:
Ergonomic Work Station, Standing Desk, How to make a vertical, ergonomic (tie-fighter) keyboard, and Create an Ergonomic Standing Desk and Office on the Go.
Step 1: Measure and test the height
I tested the setup with milkcrates for a few weeks before refining and committing to a final height.
Step 2: Modify existing legs
Step 3: Reinforce
Bend approximately 2 inches at each end of the steel to 135 degrees and drill two holes. Screw the steel to the desk and the legs forming a triangle with each leg. Triangles are much stronger than rectangles because to change the shape of a triangle you need to change the length of its members; a rectangle can change shape just by changing the angles between its members (making a parallelogram).
With the cross-bracing my desk feels rock solid.