The first thing I did was brainstorm as many science themes as I could and assigned them to different months that seemed appropriate. For example, I started with body parts because this is pretty easy (and an important concept) and my students would need to know body parts before working on the 5-senses (which we work on in April).
I came up with 9 vocabulary words for each theme and created a matching sheet and writing worksheets for the vocabulary. Then, I started searching the internet for worksheets related to each topic. I used a lot of enchanted learning and boardmaker share worksheets. For everything I didn't find online, I created my own. For the first year, I focused most of my worksheets on learning the vocabulary associated with each topic. Once my kids got this down, I created worksheets that covered actual science concepts and learning facts about the topics. So, for each topic, I am now able to create an advanced packet and beginner packet.
Each month, it is now super easy to set up our science unit...just make some photo copies of packets and pull out a tub of activities!