Start with https://www.act.org/content/act/en/products-and-services/the-act/test-preparation/description-of-math-test.html to see which topics are covered.
Always read the entire question so you know what to solve for.
Take advantage of the given answers to save time. Try to eliminate obviously wrong answers, backsolve, estimate if possible.
Practice, practice, practice. Don't peek at the key until you finish. If you aren't sure how to solve a problem, there are great walkthroughs on YouTube.
Use the scoring guide and answer key as a tool. The key breaks down the answers by reporting category so you can find your areas of weakness.
Start with https://www.act.org/content/act/en/products-and-services/the-act/test-preparation/description-of-science-test.html to see the three types of passages.
Think of it as a reading test with science information.
Look at the passage and any graphs/data tables, but don't read everything. Just read what you need to in order to answer the questions.
Save the Conflicting Viewpoints passage and questions for last.
Practice pacing and prioritize which questions to answer first. It is easy to run out of time on the science test.
Watch out for most likely/more likely questions. These require an educated guess such as extrapolation or basic science knowledge.