Writing a series rather an a standalone book is usually going to be more complex since you have more material to work with. Back when I had written the "Deck of Notes" trilogy, I had substantially a harder time than I do now when writing the "Infectious Series." So, how do you make a series easier to write?
A major tip is keep notes. This can be done in the form of outlines, just jotting important and minor facts down, whatever way is easiest for you. For me, I like making simple timelines. Each point on the timeline has little notes written by it. Usually, they're less than a sentence. That way, I can keep the order of events in check and refer back to them. I also jot down details here and there. This really helps with minor pieces of information and keeping it consistent. If your character sets an item of theirs down somewhere, write that location down so that you don't forget it later and scramble to find the information. Such a scenario was a huge problem for me when I was writing "Deck of Notes" because I didn't keep notes on the side until I edited the books.
Another important tip is gauge how you feel towards the end of the book you're writing. Do you want to add another book in the series? If you fall out of love with the story when you're done with the first, second, etc. book, don't continue or take a break. Otherwise, you might get writer's block, and the quality of your writing for the series might deteriorate. Write a series because you want to, not because it's being demanded of you. Or, the joy of writing might fade too.
As for a last tip for today, have the setting of the next book planned out as you're finishing the first book. This allows you somewhat more of a perspective on whether or not another book in the series will make sense. Have you already used all of your options, would another setting clash with the settings of the previous books and other such questions should be addressed. That way if the next book seems out of place, you know so beforehand and not when you're in the middle of writing it. Also, this lets you plan out the ending of the current book you're working on better. You know what ending to give the book so that you can transition smoothly into the next one. Anyway, I hope that this helps!