I don't know about you, but in mine school, we did a lot of summaries. Summaries of todays reading. Summaries of todays chapter. Summaries of the book. Summaries of the results of the test.
Summarising means saying the same thing with different phrases. All the while, trying to express the ideas in as little space as possible. Trying to express the same ideas and concepts in a shorter text. Making the whole text concise and dense.
But summary is not as effective, when a person uses paraphrasing, as it is when explaining with your own words. That means, that in order for summaries to be in any way effective, the concepts have to be understood, so the words used came from inside the head, not from the book, lecture,... Saying the same with different words, but not understanding is still parroting, not learning.
They seems to be more effective when used as a way to understanding than as a mean to the end.
What I am trying to say with it is, that summarizing can help with getting the big picture of the text, but they are not really helpful with remembering details of the text or making high level connections.
Also summaries are only effective for information, that was summarised. It doesn't help with remembering any other facts.
Now, if you want to practice summarizing.
Pick a text and read it.
Find the concepts from it, that you want to remember.
Write about these concepts in your own words.
I usually don’t advise to use this tactics, but that it because it doesn’t help me. If it helps you, that stick with it.
The good examples of summarizing are found on wikipedia. Just go on the wikipedia site of any series and look for the list of episodes. There are usually summaries inside (example for The Person of Interest or Sherlock).