The way you remember is you take your recall context, extract all the recall cues, and associate out from those concepts until you hit the target memory. Obviously this is a gross oversimplification, but it's basically a graph breadth first search. To make things easy to remember, you have to link them to as many other nodes as possible, and you have to link them to nodes that are strongly linked to the rest of your semantic network.

So, the best cards you can make are cards that refer to concepts, internal to you, that are highly associated. What I try to do is encapsulate the statement that originally made me understand the idea, which is usually a metaphor or analogy for another concept I've deeply overlearned. Then I make a few other cards that elaborate on the concept to get away from the metaphorical link and into the specifics.

See also: https://www.supermemo.com/articles/20rules.htm . The rules that are related to what I describe here are 11-14. This is written by the godfather of spaced repetition.