I'm assuming you're thinking of rec diving since you mentioned using a computer.
Take your table of choice (I use this:
http://dir-diver.com/en/download/ ). Write down the maximum allowed bottom time for all depths in your wetnotes and try to remember them if possible.
Make your first dive and during the dive check your time and depth now and then and try to memorize them. If you diving a square profile it's easy. If not you're probably starting deep and moving up shallower in which case you can use your average depth. It doesn't have to be super accurate - deco theory is a blunt tool.
Anyway, for the second dive you take the depth from the first dive, the bottomtime and the surface interval and put it in your table. Calculate a new set of maximum times and write them down. If you don't remember the times you can take up your wetnotes and look at them during the dive. Or write them on a wristslate if you like that kind of stuff.
And then you carry on like that. What you will see in your table is that you are pretty much clear from nitrogen after a couple of hours. So you'll start over from the beginning again.
Also after you've been diving like this for a while you will notice that you often end up with the same numbers. Mostly because you're diving the same size tanks, profiles etc. Also remember that a minute here or there doesn't matter. Just check the variation on different brands of computers and how much time they say you have left
What does matter however is how you dive, how you feel and your activites during and after the dive. I think Dr Deco wrote some good stuff about that.
And that's the big advantage of using tables. Besides making you more aware of your profiles you can also adjust them according to workload, cold, fitness level and whatever you can think of. As you learn more you can incorporate that into your profile. Maybe throw in a couple of deep stops on the way up, ascend ultra slow the last 20 feets and things like that.
Gasplanning is also a breeze because you already know what you are allowed to do (depth/time) before you actually do it.
The downside is that this is not for everybody, just like cave diving or tec diving.
The upside is that all you ever need is a $99 bottomtimer and if you start with technical diving, you already have a lot of good skills and habits that you are going to need.
Best,
Peter Steinhoff