room treatment, where do you start?

an analysis of the room would seem to be the place to start, right? what tools do you need, what do you do first? Buy, or can better tools be rented?; One presumably wouldn't need to use such tools again until you move or change speakers. Do you first calculate the two fundamental room modes mathematically and buy devices to attenuate those? I last tried electronic eq in the 80's, which introduced an undesirable reduction in clarity (tone controls were still popular, then); are analogue eg units as useful as one would expect, substantially better than they once were (I am not interested in taking an analogue signal from LP through an a/d to d/a conversion). I am starting from scratch, understand tuning by ear will be required, want a more or less scientific, targeted approach. It's a rented apartment, so it's impractical to invest in experts, e.g. Rives.