You can combine one or two sentences so that you get shorter sentences.
Use one word for several words.
Do not just copy and paste the content. Try to rephrase it.
Omit all the examples, illustrations, quotations, figures of speech and any ornamental language from the original text.
The most common thing observed is that learners often add their own interpretation or opinion in summary. Do not do this. The summary should be a reflection of the original text but in brief.
Title
Giving a suitable title to the summarized text is important. To find a suitable title, identify repetitions. If certain words are repeated throughout the text, it means the whole paragraph is related to that word.