An argument by analogy always follows this general pattern:

PREMISE: Two things are similar in some way(s)
CONCLUSION: Those two things will be similar in some other way(s)

The key to strengthening/weakening any argument is to strengthen/weaken the underlying assumption.

Once you've identified an analogy argument, you immediately know what the assumption is:

ASSUMPTION: The similarities in the premise are relevant and sufficient to explain the similarities in the conclusion.

For example, let's say I made this argument: "Grapefruit and oranges have a similar peel, pulp consistency, juice concentration, and even color and size. Therefore, if you're looking for a good source of vitamin C, you'd do just as well to eat a grapefruit as an orange."

The assumption is that peel/pulp/color/size/etc. are somehow relevant and sufficient to explain Vitamin C content. To strengthen this argument, you'd cite some sort of evidence that in fact Vitamin C in a fruit is produced in something the two fruits have in common, for example pulp. To weaken the argument, you'd cite something to suggest that Vitamin C content in fruit has nothing to do with any of the similarities cited in the premise.

You can learn the approach for the other argument patterns here: https://www.dominatethegmat.com/video-pu ... ng-part-2/

I hope this helps!