Fill it with water from your tank. Some sort of filtration will be needed (at least a cheap mechanical filter), and a heater of course. A few places for him to hide are important.
Biofiltration will be an issue since this tank is not cycled. You can put a few pieces of LR in there or add a sponge from your tank/sump to get that good bacteria started. Whatever you do, expect to do a few gallons of water changes every day and monitor ammonia closely in the beginning. I don't know about Amquel Plus, but if it's being recommended here it is probably a good product to control spikes.
Don't be tempted by reef-safe treatments, a real QT is the best way to take care of this problem.
If your fish is in real trouble, do a fresh water dip prior to adding him to the tank.
You can do hyposalinity or copper treatment. Remember copper is toxic to inverts, so keep it away from your DT and make sure you don't put things from QT in DT unless they are copper-free. You're looking at 6 weeks of QT to let the ich in your tank die so you wont' get a recurrence. Don't be tempted to put your fish back in once you think it's cured, it stays in the tank longer. If you want to do hypo, you need a refractometer (Not a hydrometer, they aren't sensitive enough), and you need to lower the salinity over a few days.
10 gallons is small for a hippo, but I think it's fine unless he's a big guy.