As far as I know, there are no readily available guides for SMK speedruns. Most SMK runners come from the Time Trial community, where they already have years of experience playing the game competitively.
That being said, the run is fairly straightforward: play through all four 150cc cups in order with no console resets. There are no sequence breaks or anything like that. The only thing you need to learn to start off with is basic driving (which is actually really hard and has a lot of depth on its own). Then you can start adding layers with more advanced stuff like item-based shortcuts and whatnot.
Go to the virtual console menu on the gamepad and in the top left corner you should see four squares in a horizontal row with the first blue square illuminated designating player 1; tap on the touchscreen to change to player 2. You'll have to drive on the bottom screen though.
