How could I possibly apply rule-utilitarianism in real life?

Typically, it will be enough to imagine one or two rules and consider the utility of the consequences if you would follow these rules, generally.

The textbook example is the broken promise. Imagine, you made a promise to pay a kid 10$ to mow your lawn. After the job is done, you consider giving the 10$ to charity where it would be passed to a child much more in need and therefore where it would generate much more utility.

According to rule-utilitarianism, you would have to compare something along the line of the utility generated by the consequences of following the rule "keep promises at large" vs. the utility generated by the consequences of following the rule "keep promises at discretion".

Of course, which rule exactly to test and how to formuate it, is hard to pin down. But that is notoric in ethical problems and solutions. I find rule-utilitarianism an idea that can be put to praxis.

Do I need to now make thousands of rules...?

It would be impossible to make all the rules you need. From Sheldon Kopp's Eschatological Laundry List:

We must live with the ambiguity of partial freedom, partial power, and partial knowledge.

All important decisions must be made on the basis of insufficent data.

Therefore, a good course of action might be prioritization. Try to brainstorm some of the more important rules for situations you are likely to encounter. Under what conditions would you want to run a red light, lie or kill someone?

If you run into a more unusual situation you aren't prepared for, then you just have to sit down and study that situation.

That's pretty much how law works. There were no laws governing patents or the Internet until the advent of patents and the Internet.