In addition to bfv's link, I found this guide pretty informative and straightforward.
I think the best thing to do to get used to drawing in isometric is to get some isometric graph paper, or just print some out(PDF warning), and just draw a shit ton of cubes of varying dimensions. I mostly just searched through reddit.com/r/Isometric until I found something really basic looking, and then I just tried to copy it on graph paper. I also tried to model basic furniture around me - mostly tables and stuff.
Understanding depth in isometric grids is tough, and I mostly just draw lightly with pencil until it looks right. Because it's modeling a 3D space, I find it's pretty easy to see when something is going the wrong direction (granted, it's sometimes less easy to figure out where things ought to be going...).
The only thing you really need to get started is a triangular protractor, which should have a 30 degree angle on one side. If you have a transparent ruler and a better knowledge of geometry than me, then you may already be fine. You just need some way to make 30 degree angles.
I'm planning on doing some more paintings this weekend, I'd be happy to take some pictures along the way and write up a quick visual guide for how I do stuff. There's a bunch of tiny little tricks I've picked up along the way, I'll put them in there.
Generally speaking, I make a cube, then an isometric grid inside of it, both in pencil. I draw some furniture, erase the gridlines, then border everything with a fine black marker. After that I fill in with acrylic paint and some markers. To tell the truth, I know nothing about painting theory, so I just make most object monocromatic, and don't clean my brushes too well. Sometimes it works out well.
I'm not entirely sure yet what these rooms will be - some of them are idealized rooms from my memory, some are rooms I'd like to build some day. I think they're all part of one mansion, big enough that you can sleep in a new room every night. A nice place to visit.