They say you can guarantee a today but not a tomorrow, what should I not do today to guarantee a tomorrow?

You can be safe today and think about tomorrow and plan for tomorrow and get ready for it.  All of which you should do!

But the point of the saying is that tomorrow is never guaranteed. Yesterday is a memory.  Tomorrow is a promise.  Today is a b*tch!

The only thing I can think of is.... don't die? Then tomorrow is guaranteed. But they also say that that tomorrow never comes, because today is the tomorrow of yesterday. So you technically never really get to see a tomorrow since when we do get there its no longer referred to as tomorrow but instead has turned into today.

In all seriousness, I think what you're talking about is actually trying to make the point that you should live your life to the fullest and do the absolute best you can in everything you do today because literally anything can happen between now and tomorrow and you don't want regrets, basically.

You have it all wrong.  You could "wrap yourself in cotton wool" which makes it more likely that you will reach tomorrow. But as you have then missed out on TODAY it is a losing strategy.  If you live today to the fullest then it doesn't matter so much if there is a tomorrow or not.