So how do I actually do this?

forum system.

Use the green "Open" in the top right corner to edit the status, category, or title of your thread, or to edit the initial thread post. :)

What's the process of making sure that I can give those codes without binding them as "for myself"

The giveaway creation page accepts Humble gift links. Given Humble's policies, it's advised you only give away gift links, rather than steam keys, when giving away from Humble. (Humble can reset keys and gift links back to a neutral state for you, on request, so that you can select the other option.) When interacting with any bundle site in regards to your usage of sites such as SG, it's best to omit mention of where you're using your keys, as many sites have policies against such usage, despite such usage being legally permissible in most regions.

As noted below, you can freely add in keys after a giveaway ends, even if you didn't add one to start with. Just keep in mind that winners have up to a week to claim their win (so be careful of any sales expiring, if you don't already own the key), and that Steam will not allow you to gift to regions with currencies that are over 10% higher value (which essentially means people in the US can never gift through Steam to those the EU). You can utilize http://www.SteamDB.info to check comparative prices for any given game (as currency differences may change game to game).

this system here

You can utilize http://www.sgtools.info to check the status of someone's SG account (though it's against site rules to call someone out publicly over any sort of infraction, so keep that in mind). It's advised to check a winner's account for infractions before sending your game to them, and to put in a Request New Winner support ticket if any infractions exist.
If the infractions have already been punished, and it's not within one month of the most recent punishment, then your reroll request will be denied; On the other hand, you can freely choose to give your game to a winner despite any infractions they may have, and utilize a user report ticket to inform staff of any issues.

If you already have the game you're giving away, there's no need to choose the Gift option. Just choose Key and paste the giftlink / Steam key right away

Did not know SG recognised HB giftlinks as keys. ✌

Most games aren't generally region locked, normally the bundle will say if it can't be activated in a certain region.

But yes, it's something you have to manually do - though I've never had to with any of my games.