@Alwayspainting and @Norsk and anyone else. This is not the only way to do it - if you ask 10 different people how they create a unit, you’ll get 10 different answers. My answer is kind of a combination of what other people have said.
- I begin with a template for my unit/lessons. I break the unit topic down into subtopics that would be suitable for each lesson (ie - Unit may be “Astronomy for First Grade” - Lesson 1 - Sun, Lesson 2 - Constellations, Lesson 3 - 8 Planets, etc - I just break it into manageable topics for each lesson.) I usually aim for a month long unit - 2 lessons per week, for 4 weeks (give or take)
- I put the topics into the order I want to teach them. I take lesson 1 topic (Sun) and I Google, Pinterest and TPT the topic. Sometimes I want to make the unit from scratch and other times, due to time, ideas or interest, I want to grab something off of TPT or a blog for free or little $$. “Don’t reinvent the wheel!” - if someone made a great unit and it doesn’t cost much (or even better - it’s FREE!) then work with it
- If I can’t find a premade unit or feel like making it myself, I research topics for each lesson. Depending on the age of my kids I’ll choose 1-3 experiments/crafts/projects for each lesson.
- After I have chosen my activities for each lesson, I make a list of pertinent vocabulary words (usually 1-3 words) to go into our science and history dictionary
- Totally optional: next I go to my library website and look up each topic to make a list of books that are age appropriate for the unit. I make my units months and months ahead, so this works well so when it’s time to do the unit, I can easily take my list and grab the books.
- Next, if I have papers to print or lapbooks to prepare, I do it now. Even if the unit won’t be taught for several months I do it now so I can store it all together - then it’s easy to pull out later. I’ve found that if I make my unit ahead but don’t prep the paper stuff, then sometimes I forget what/how to prep it when I pull the unit out 3 months later.
** OUR FAVORITE PART: We have a History/Science drawing journal. At the end of every UNIT (NOT the end of every lesson) my kids draw a picture of the unit - whatever they want - then on the back they write a few sentences about something they learned in that unit. At the end of the year they have an entire drawing journal of all of the science/history units we learned that year. My kids love this, and I do too. There is no way we can keep all of the experiments and projects from over the year, but we can keep these beautiful drawing journals.
I don’t always make things from scratch, I don’t always buy things, I don’t always use lapbooks, and I’m not always organized. I just do the best I can. I figure if nothing else, if I manage to have a pile of books on the topic and a few crafts/activities and a writing piece here or there, then I’m golden 
The rest is all cherries on top.
BTW - If your library carries them, another great resource for science units are the Mailbox magazines - they have TONS of them - some by month, but many by topic - plants, insects, oceans, weather, astronomy, etc. They come in different age levels - full of crafts and experiments, book suggestions and reproducibles. You can buy them also, but I’m cheap