Start slowly and build up. My desk is electric so I can raise and lower it when I want. At first, I just stood for the first hour of the morning and then another hour at the start of the afternoon. Then made sure that "hour" got longer and longer as the days and weeks passed.
What also helps is moving around rather than being stood still for hours. I'd previously been "saving" trips to other areas of the office, i.e. the filing room, copier, printer, loo, kitchen, etc., and heaving myself of my chair once every couple of hours to do loads of things before sitting down again. So very long periods of just sitting. When I was standing, as I was already "up", I'd just trot over to the copier, printer, filing room, etc., many times per hour, which made me more active in addition to standing rather than sitting.
My step counter previously showed just a few hundred steps per day walking occasionally around the office, but once I was standing for a couple of hours in the morning and a couple of hours in the afternoon, my steps increased to a few thousand per day, without me even realising it.
Being more active throughout the day, on top of standing when working at the computer, certainly helped with hip pain. Nothing worse than standing still for a long term, but that's really not the whole point of the standing desk - yes, it improves your body "stationary" position, gets the blood pumping more, and builds muscle in the backs and legs, but more important than that is the way you're more likely to move around more in your workplace simply because you're already on your feet, so going to the copier isn't a big effort anymore and instead of returning/collecting several files at once, you go multiple times to put one back and take the next out.