It’s usually just a matter of lubing up the o-rings a bit with some silicone and screwing in the hoses to the first stage. I usually start threading them in backwards, until I feel it drop into the grove, then proceed in the other direction slowly. If you feel any tightness, stop and back out and try again to make sure you’re threading it properly. Once hand tight, then get a wrench that fits well and tighten until nice and snug. If you’re not comfortable with that, take it to a dive shop.