ELI5 Houston: Taking the Metro

I feel like a big ol' dummy for asking, but in 24 years of living in Houston, I've never taken the Metro Bus, and now that I need to, I have no idea how.

I understand you buy a local 3 hr ticket, or an all-day pass, but how do you know when to get on/off, which buses to take? I've tried using the app, made me more confused.

Is there something I'm missing, or am I just that dumb? Somebody help, please and thanks!