1.
Suppose you have to diagram this argument:
[¹The medicine was effective];[² the patient recovered].

Why is

1
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2

wrong? Isn't the effectiveness of the medicine the cause for the recovery?
Answer
Right. But there's a difference between A being the cause for B, and the statement that A occurs being the evidence for the statement that B occurs. In the example, the evidence that (1) is true is given by the truth of (2): I know the medicine is effective because it cured. Similarly, although fire causes smoke, and hence is temporally prior to smoke, I may know that there's fire because I see smoke.  That is, although the causal flow is from fire to smoke, the evidential flow is from smoke to fire. So, the correct rendition is:
2
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1

2.
Is there a general strategy that allows me to diagram with ease?
Answer
Unfortunately, there isn't.  However, here are some suggestions:

• try to understand what the argument is about and what it says; if you don't understands that, it's very hard to diagram the argument correctly.
• determine what the conclusion is.  If you err on this one, you're in trouble.
• make sure you circle all the evidential phrases (therefore, hence, so, etc.) and use them.
• if you locate subarguments in the argument, work on them first.  This will allow you to move your attention away from their premises.  With fewer sentences to look at, finding the flow of evidence will be easier.
• practice.  Diagramming is a skill, and skills can be mastered only with practice.