I have a Calculus textbook. Which is more effective ? make my own notes, or spend all my time doing exercises ?

I'm a student of electrical engineering. Believe me, in-depth comprehension of calculus is prerequisite for understanding physics of electromagnetics.

You start by reading about the matter, for example, what are partial derivatives and why do we need them. I found out that by doing exercises and making notes whenever encountering new theoretical concepts greatly shortens study time. If your professor is known to expect students to understand theoretical aspects of calculus, learn them afterwards doing exercises. You will have a different perspective and a bit more knowledge than if you would just be reading a boring textbook.

You should seek to find what works best for you.

Do not be afraid to re-read the sections, and DO NOT be afraid to reference the section to help with questions. Don't be like the student who sits there struggling on a problem because they don't want to "cheat" by re-reading the topic. How else will you learn?

Making your own notes is fine as well, especially if you re-word it since that may show a deeper understanding. Obviously difficult for formulae and such, but maybe you could describe what is going on, or what fundamentals make it work, or describe it in "simpler" terms, as if you were trying to teach someone who didn't have as much mathematical experience.

Spending times doing the exercises is never a waste of time though, and will definitely help aid your understanding of the topics!

A good compromise is skim read the derivations/theory and then look for worked examples in the text.

This helps to see how otherwise confusing notation is applied in practice.

Many textbooks print the main formulae for a given topic in heavy print, so be sure to note those.

Try to rework the given examples without looking at the text and perhaps then you will be both

a bit more informed and confident when working through the set questions at the end of the topic/chapter.

If you get stuck or do not understand a particular topic do not be afraid to ask your teacher/professor about it.

I would suggest you spend the majority of your available time doing exercises. Short notes under topic headings will aid you in doing exercises.