Like Backpacker I start with the manual. There can be lots of interesting 'gotchas' and you need to know them. Sometimes it's something embarassing in normal ops but also potentially life threatening such as how to operate a door from the inside, to more complicated things like the electrical/hydraulic/pressurisation/geared engine systems that must be operated correctly.
Even in simpler types there are things you need to know that the manual is there to provide. A little while ago I was replacing a leaving pilot at a dropzone using a C205. He'd been flying it for nearly a year and couldn't work out why performance wasn't very good for take off or time to altitude. It's a grass-on-sand airstrip with lots of loose sand patches so soft field method would be a good idea. Once airborne, time to altitude seemed sluggish.
First thing I did was read the manual - even though I've flown C205s years ago - where it clearly gives a soft field take off technique (nothing unusual, but it's there) and Vy for various altitudes so easy enough to interpolate to derive Vy per 1000'. I flew the aircraft and could get airborne with a greater load & get to altitude marginally faster. He'd never read the manual & was ignorant of those things.
At the same field an inexperienced pilot crashed a C182A through fuel starvation on short final. He was very, very lucky to escape with minor injuries instead of drowning in a lake in the undershoot area. A read of the manual, and particularly a study of the fuel system schematic, might have shown him the inadvisability of a highly slipped descending turn with minimal fuel on board.
I'm taking over management of a group owned C421C - and here I've been for the last few days with the manual in front me even though I've flown a C421 before and other C3xx/4xx types.
As for leaning: Lean at any & every altitude that the aircraft's power setting allows. 2000' or 20,000' makes no difference.