Even though there are different materials of candle covers (plastic, paper, resin, etc) the process to trim them is really the same. I recommend using a hacksaw. The tiny teeth and firm grip give you the right balance for a smooth and even cut.
Next, find an object to place in the candle cover to support the shape while you cut. We sometimes use a nut driver. Do not use your finger.
With support in place, you can start to hack. Remember to push and pull the saw and let the teeth do the cutting. Too much downward pressure will warp or smash the candle cover.
Now the candle cover is the right size, you will want to clean it up. Here you can use a knife, rag, bench grinder... whatever you prefer.
That is easy! You can use the same process for paper candle covers:
And resin candle covers:
It is just that easy.