I had to plan this modification from the start, before even assembling the new IKEA Galant desk. Since the desk was so large and I couldn't really flip over the frame, I had to attach all items, including the pegboards, while laying on my back. I had to drill holes in the pegboard for the legs and the areas where I'd be screwing the desktop to the metal frame. I'm no construction pro so my larger holes in the pegboard were sometimes off, but it all worked out in the end. I attached everything - including the pegboard to the metal frame - with medium and large zip ties.
I believe I spent a good 6+ hours on this project, but I'm really happy with the results. I even installed a USB hub near the front of my desk so I can plug in extra equipment easily.
When it's all said and done, I spent only $30-40 on supplies (zip cables, 2x4 pegboards (x2).
Step 1: Attaching the pegboards
Attaching the two 2x4 pegboards is a bit tricky and you'll find yourself on your back often. The trick is adding the pegboards into the construction of the desk AFTER completion of the desk's metal frame but BEFORE the legs are attached to the frame. Larger holes need to be made in the pegboard where the leg attaches to the frame from the bottom and where the tabletop is attached to the metal frame from the top.