How to draw lewis dot structures...?

I'm having a hard time drawing lewis dot structures.
though I get them correct, i have no idea how i got there.

for example, i know that ClF2 is F-Cl-F, but i have no idea how i did it.

same thing with O3, I know its got a resonance structure, and I know how to draw it... but I've only survived the last several days of AP chem by memorizing it, not by knowing how to do these.

can I have some basic tips when dealing with these?
 
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