What is the best way to start a sequel? Would you recommend including a short summary of the previous story or is it better to just start from where it ended?
It depends on where you left off.
For one of my series, I did a sum-up of book one before going into the second since I was finishing up an event that had happened at the end of the first book.
On another of my series, I could start of fresh into a new story arc without new readers being left in the dark of information from the first book.
A better question to ask is if the story needs one. I know a lot of stories that end up falling apart when a sequel is written because there is not enough to write from. I think if you are going to write one though, it is best to analyze your characters, their journeys, and be sure you can continue them.
There are multiple ways, off the top of my head
1. Direct continuation
2. Sequel about a different character
3. Time-skip sequel