I used index cards to help my kids understand how to outline. for example, as they read a paragragh the need to first write the main idea of the paragragph, then look for details to support the main idea. We did that with 3-4 paragraphs,with guided practice. Today they are able to outline when I ask them to but, of course there are some that still struggles with it.
Textbooks are usually divided into sections. Each section is a Roman Numeral. The main headings under each section are the Capital letters. Once we have that established, then we go back and find details. At first I tell them how many details I am looking for under each heading. We are now at the point where they do not need that any more.
In the past we are usually able to develop out own outline of ideas for a report by the beginning of second quarter.
The most difficult thing I find they have is not making the outline in a straight line down the page.
I usually give them a copy of something and they actually cut it apart and put the important parts into outline form - with real glue. It's a great activity - and works the opposite way too - give them cut text and have them put it back together.
Use a graphic organizer to get them used to all the bits and parts. When they are really familiar with that, transfer the information to a formal outline. My school uses the Four Square Writing Method. Transferring from that organizer to a formal outline is much easier this way I have found.
I have been doing outlining as well, and started out with them helping me outline character traits of Donkey (from Shrek) on the board, then I asked them to write an outline about themselves. Most of them got it on the first try. After that, we talked about the numbering and capitalization particulars...and I have them outline their guided reading books for practice.
Runsw/scissors is right. I have found the graphic organizer a good way to ease into outlining because many people--including adults--hate the strict formality of outlining. Another technique to ease into it is to do brainstorming--a mind map--that will allow one to get all the information out on a page almost categorized and then you can move on to outlining it. Brainstorming is easy to teach and use. Give it a try.