Leeching is very simple.
Basically you join an official FC, like "keyers" or "sell dung", and you look for ranked members within the friends chat. These are the Keyers, or the people who will be keying the dungeon for you. Basically, once a keyer announces that they are open to take customers, you must tell them what you want and they will tell you the price. For example, you want an Occult floor and they say that it will be 3m.
The keyer assembles a team, usually of 2-3 suicide dungers (guys who will repeatedly die and do whatever it takes to finish the dungeon as fast as possible) who are very experienced, high-levelled keyers, and the other 2-3 members being paid leeches. As a leech, you pay the keyer who is taking you. The keyer creates a floor and takes the team inside the dungeon.
The rest is extremely simple, as you literally do NOTHING, except stay semi alert with your screen open, so that if there is an emote room or the room of levers you can gatestone teleport immediately to assist the team with that room so they can move on. Alot of keyers offer a refund time too. For example, most of the keyers in "keyers" FC offer a sub 25 refund I think it is? Which means that for anything above 25 minutes, you will get your money refunded. The dungeons are extremely fast, extremely expensive, and extremely good exp
Hope that helps a bit.