Personally, my first thought about an allusion is that it is a connection between one work and another. So maybe you can draw some sort of diagram that shows how the work you are reading and the work that it alludes to are connected. Of course, I doubt you'd get much credit if you just draw a connecting line between two books, so you'll have to do a little deeper thinking about how exactly they are connected, but that's a starting point.
Maybe try to think of a metaphor/simile for an allusion - what is an allusion like? Try to think of some physical object that connects things that you could compare an allusion to.
Again, there has to be more to it than just the fact that it connects things. You'll probably have to be able to explain to your teacher why you picked that type of connector over another. So try to think about how the two works relate to each other. Is one more important than the other? If so, draw something that connects a less important object to a more important object. There are some types of connectors that send information two ways, like a phone line, where people on both ends can talk. An allusion doesn't go two ways, so you probably want to avoid drawing something that connects things in both directions. Start by brainstorming a big long list of things that connect two objects, and then try to think of more specific ways that each item on your list might be similar to what an allusion does. Pick the one that fits best.
I know this isn't very specific, but I hope it gives a start, anyway.