The first step in getting dressed is to put on your compression/wicking under layer. I use a one-piece system but there is a variety of combinations out there so you can mix and match with whatever you like. Some players like to wear some compression under layers that can help them cope with old injuries. The most important part is to be comfortable.
Once you have that under layer on, put on your athletic support (Jock strap). Make sure it is positionned correctly, that it will not move out of place and that it appropriately covers your genitals. Tighten the leg straps and double loop them to ensure they will not become loose with movement. The last thing you want is a shot straight to the groin with no protection!
Once your athletic support is properly positionned, put on your pants. The system to hold the pants up varies per model but usally features a lace and some straps. You may also purchase some suspenders especially if the straps will not keep the pants in place. Goaltenders usually try to get the biggest equipment possible in order to cover more room (without affecting agility) and so your pants may be much bigger than your body. In my case, my pants were purchased when I was still growing and now require no extra support system.