You have several sub-skills for learning how to read. Depending on the type of text a student is reading, the student may look for the gist of the text, or the student might look for specific details. For example, a student might scan a news article to get the gist of what happened, but it would not be wise to only focus on the gist of a court summons.
I have used many skimming and scanning exercises in my classes and have found that it is usually very difficult for the ESL learner. Mainly because they have not learned how to pick out the keywords in a sentence. To them, they are all keywords.
I had to teach my students how to scan before they could do it well. I did this by giving them a story with only the keywords visible in the story. It was easy to find the gist of the story. Then I gave them the same story with all of the words and the keywords highlighted. My students quickly found the value of learning how to scan.
We also did exercises where they had about 1 minute to scan for keywords and underline them. I also had a paper with underlined keywords and whoever got the most correct keywords underlined got rewarded for it.