What's the grammatical structure for "  like a " style simile in Chinese
This is indeed a complex matter. From the way you ask, I guess you're interested in the grammatical aspect, which is what this answer is focusing on.
Because Chinese language uses serial verb construction, the simile is expressed as a V-O (verb-object) clause with simile connective as verb and vehicle noun phrase as object.

[simile clause] = [connective verb][vehicle noun phrase][optional simile suffix]

Simile connectives include: 像, 好像, 好似, 如同, 仿佛, etc.
The suffix is an optional part to emphasize the analog. Words include: 一样, 一般, 似的, etc.
Choosing connective and suffix are highly contextual and idiomatic. I don't think there are simple rules. However, connective 像 and suffix 一样 are applicable in most cases. When in doubt, go with these two.
Because there is no ordering relationship between the original verb clause and the simile clause, they can be placed in either order:


[subject][original verb clause][simile clause]
[subject][simile clause][original verb clause]


A common realization with 像..一样 is:


[subject][verb clause]就像[vehicle noun]一样
[subject]像[vehicle noun]一样[verb clause]


Using your example,

The light flickered like a candle.

Can be translated into

灯光闪烁不定，就像烛火一样。
灯光像烛火一样闪烁不定。