How do you do it?

First thing first; Trading is a massive pain in the ass, and you should know that.
Yes, it is rewarding in the end. But it requires lots of work, time and efforts.

Through the scammers, beggars, lowballers and raging traders, you'll have to buy and sell games.
I'd recommend starting with purchasing bundles for cheap, and re-selling them.

I'm personally not much of a trader, so I'll let the pros handle the rest.

Multiple questions, multiple answers(in no particular order):
-How to get higher levels:
a)In steam: collect trading cards sets and then turn them into badges.
b)In steamgifts: create giveaways.
-How to get hundreds of games to trade(in gift form): buy games during sales when they hit their lowest historical price. This site can help you.
-How to get thousands of games in your account: buy bundles and games on sale and you'll be there in no time. Warning! Is addictive.

You'll need money for all these things, so get that wallet ready to be raped.

"wonderful world of trading"

yeah, okay, heheheh

It's not as easy as it was when I started. Now all newly purchased gifts have a 30 day non-tradable period, which would then require that you have either a good reputation or go first in trades.

Start by having a job and some disposable income.

Stay out of Steam trading if you know what's good for you.

No, seriously. Don't get into it.

Hack Volvo database.

With everything that has been going on lately maybe it's a good idea to not even start =/

Obviously need money to start. All those items people trade away aren't free. The trick is to find something you can get cheaper than other people from other countries and trade it at a profit. Being in the US, you won't have any of the region locking that certain countries now deal with, but you also don't have anything special going for you in terms of prices. Here in New Zealand, I've been able to purchase games on price errors in the past and trade those away.

Lots of alcohol. The rest takes care of itself.

It's probably people throwing lots of money into steam and then buying everything during flash sale.

That, and constantly checking the many bundle websites out there.