Can you give me an example of one you find tricky from one of your practice tests? I'll break down a simple example question from a prep course website. I'm going to explain pretty in depth--not trying to be pedantic, I'm sure there's stuff you already understand, just don't want to leave out something you don't.
Argument: All sharks have fins and all dolphins have fins therefore they are similar.
Answer choices:
(A) Sharks and dolphins are similar, however, they have many differences.
(B) All sharks have teeth, dolphins are similar to sharks therefore they must have teeth.
(C) Bats and eagles must be similar because every bat has wings and so does every eagle.
(D) All dogs have eyes and all cats have eyes.
(E) Some rats have fur and all mammals have fur, therefore, rats are mammals.
I would write the argument's diagram like so: SF (sharks have fins) & DF (dolphins have fins) => S~D (sharks and dolphins are similar). That's two premises that together lead to a conclusion. This arrow, =>, you use when you see "therefore," or something similar, indicating the conclusion. This arrow, <=, you use when you see something like "because" or "since," indicating the reasons for something.
What I would do during the test here to save time is first eliminate any answer that doesn't have enough elements--so D, which has no conclusion, would be out (that diagram would be DE & CE). Also take out any that obviously contradict or expound on the argument, like A, which just introduces a new idea (that diagram would be S~D & S≠D). The other three you could diagram like so:
(B) ST & D~S => DT
(C) B~E <= BW & EW
(E) sRF & MF => R⊂M (the sign I used there in between the R and M is the sign for "is a subset of"; it can also go the other way; M⊃R would mean "M contains R")
E doesn't look anything like our diagram SF & DF => S~D. The other two do, but what you want to preserve is the relationship. Your conclusion should remain the same, and it's always going to be on the side that the arrow's pointing to. That is, SF & DF => S~D is equivalent to S~D <= SF & DF. You want the "shared element" of the two groups (like fins in the original argument) to be in both the premises, and you want the conclusion to be about the two groups' similarity. B switches these around. So C is your answer. The order in the sentence is backwards, but the diagram is equivalent.
If you want to go a little bit faster, after your initial diagram, instead of continuing to diagram, try just focusing on the concepts to each part: the two premises show a shared feature between two groups, and the conclusion states that this means those two groups are similar. Anything saying "group A and B both have X, so A and B are similar" will work. If you read the sentence with variables, it becomes clear that C ("A and B are similar because group A and B both have X") is the same idea.