This is a very simple method that I use...while reading picture books to students or chapter books, I stop at points and ask a couple of students what they think will happen next. When the first child answers the questions, I prompt a student to answer who has a different opinion of what will happen. (I also make sure to tell the students who have already ready the book that they are not allowed to answer...or I tell them that they need to think of something else that could happen other than what they know is going to happen.)
I also stop and do a turn and talk, I give the students exactly 30 seconds to turn to their neighbor they are sitting by on the floor (or at their desks) and let each student take turns sharing what they think will happen next. I give 15 seconds for one student to share, then I call STOP, SWITCH, and give the other student 15 seconds to share what they think. After we find out what happens, I ask students to thumbs up if they were correct, or thumbs down if they got it incorrect. (Of course I explain that if they use context clues and make inferences that there is no "wrong" answer to a foreshadow.)
Foreshadowing is also great to introduce context clues and making inferences. If we're reading a book about polar bears and I ask the students to foreshadow, of course if they say, "I think the polar bear is going to go to a tropical island," that's not going to make sense. We discuss that as well - that they need to have reasons, and proof to back up their foreshadowing (because nowadays...kids gotta have proof for EVERYTHING on those state tests!!)
A last thing that I do, I give the students sticky notes (post-it notes) or I have the students bring their writing journals and I stop and have them write a sentence on what they think will happen next...sometimes we discuss as a class, sometimes not, and sometimes I have them share with their partner.
Every time we do this, I make sure to explain that we are making an educated foreshadow. But I think it can easily be used with every single book that you read to the kids, it just reinforces the activities and lessons that you're doing. It's great to do it with different books too so the kids get used to it.
Also, have them do the same thing with they books they read independently during silent reading time. Tell them to at least pause in 2 parts where something is about to happen, write their thoughts on what they think will happen next, and then go back and write if they were correct or not.
MAKE SURE that you teach them to back up their foreshadowing. "Well, I think this is going to happen BECAUSE..." And make sure they know how to make 'educated foreshadowing guesses.'