If I understand the process right I submit my game to "game portals" and they take a look and say yes/no etc?
Pretty much.
Any tips and general advice on this process would be appreciated
If your game is good enough and a portal sees potential to sell it, they'll want a contract detailing the specifics. they will either offer an exclusive contract (them being the only authorized reseller) or a non-exclusive one. Exclusive contracts tend to pay you more per copy sold, but means that you can't have similar deals with other portals to also sell it for you.
Payout percentages vary, but from what I've heard it's typical for them to take 70-80% or so of the revenue, with the remainder going to you.
Sounds like a bad deal, but keep in mind that they have the audience and marketing, and can put your game in front of hundreds of thousands of potentially interested people.
You may get 100% of the money if you sell it through your own site, but 100% of 3 sales is still a whole lot less than 25% of potentially thousands of sales.
also keep in mind that it will be at least several weeks before any game would make it onto the portal sites after they tell you they are interested. That's important to keep in mind if you have a seasonally oriented game: a halloween or christmas themed game can get a lot more sales if it is up on the portals before the actual holiday starts, because not many people look at buying christmas games in february...