I am getting some new tubes for my amp, and I want to do a 24-hour burn in. How?

OP, gotcha. It does change the sound, and (again) it's interesting to hear the changes.

I'm amazed at how many people are telling you to just plug it in -- as if there are not a million threads about tweaks, techniques, etc. You're asking a reasonable question that many have asked before (in one form or another) and you're getting your head patted as if you were over-worrying. This is a hobby and we're on a hobbyists' forum and you're asking a hobbyists' question. 

@hilde45 thanks man. Yeah, there are a lot of people saying different things. Thanks for clearing it up though.

so you suggest I leave it on standby for about 24 hrs?

Playing a signal through the tube for twenty minutes will do a lot more than running it idle for 24 hours. Just turn on the amp and play music.

Replacing the stock tubes on my preamp it took 15 hours of PLAYING time to start hearing the upgrade. Keeping the preamp on 24/7 without playing anything showed no improvements.

I like to hear the changes. Part of the fun. Part of the experience.