The best way to prepare for the SAT or ACT is to mimic the testing process at home, as much as possible. You can create a test packet that is  very similar to the one they will receive during the real test. Practicing with your test packet at home will give the most similar test results.

Here is how to make your own test packet. Start by buying a book of real tests. Below are the books I usually suggest.
Note: This post contains affiliate links. If you click and buy I may make a few pennies, but not enough for a latte.

For SAT Preparation

The College Board Guide: Official Study Guide 2018 Edition
The Princeton Review Book: Cracking the SAT 2018

For ACT Preparation

The Princeton Review Book: The Princeton Review Book: Cracking the ACT 2017 Edition
The ACT Company Book: The ACT Company Book: The Official ACT Prep Guide, 2016 - 2017

Take one whole test, with answer key, and slice it out of the book using an exacto knife (AKA "The Sword of Exact Zero" from the Lego Movie). Taking out one whole test like that will be the most like the test packet used at the real test. Then be ready to slice and dice that book to create a paper test packet. Here's how to use those books.

1. ​Use the same kind of pencils, calculator, scratch paper, and water bottle that will be used during the actual test. Remember that phones are not allowed - put those far away.
2. Take one section per day, 3 days per week, doing the sections in order they are presented in the test packet that you made.
   
3. Give the section timed, using a kitchen timer set by the parent that rings when time is up. Again,  phones are not allowed, so practice using a watch to keep track of time.
   
4. After taking all of the timed sections, child or parent marks answers correct or not correct.
   
5. Student reviews just the missed, wrong answers, and figures out what they did wrong.
   
6. After you have done all the sections in the test, then combine them all to calculate the score for that whole test (even though it took about 2 weeks to get through that test, it can still be a good estimate of improvement.)
   
7. Once a week, practice for the marathon aspect of the test. Again, take out one whole test with answer key and slice it out of the book with an exacto knife. Use the whole test once a week, timed, exactly as the directions say to be time. Don't take more than a 5 or 10 minute break between sections, though. Take the whole test timed, then parent corrects the whole test to determine the score for the test. Then have the child go through the wrong/missed answers to figure out what went wrong.

​Be sure to correct the whole test exactly as they describe in the scoring process, so that your practice test at home score will be as close as possible to the real score your child will get.

Does that step by step instruction help?