Sometimes you'll start a sentence with a descriptive phrase, for example:
Sprinkled with chives, [...]
Smiling broadly, [...] .
What comes after the starter phrase and the comma? Make sure it's the same thing that the phrase describes:
Sprinkled with chives, cold potatoes are loved by kids.
Otherwise you risk writing a very silly sentence. The starter phrase floats loose, unattached to its real subject. For example:
WRONG: Sprinkled with chives, kids love cold potatoes.
Starter phrases often use words ending in -ed or -ing:
sprinkled, worried, married, smiling, thinking, dozing, walking
These are parts of a verb. Just like complete verbs, they should be placed close to their subject. Don't let them float apart.
RIGHT: Smiling broadly, the CEO signed the contract.
WRONG: Smiling broadly, the contract was signed by the CEO.
Now expand this rule
You have learned the easy part. But wait, there's more! Stick any descriptive phrase close to the thing it describes, to make sure the meaning is obvious.
If in doubt, just rewrite the sentence. Don't struggle with grammar: say what you need to say, clearly and simply. Sometimes a sentence can be grammatically correct—and yet the meaning is not quite clear.
UNCLEAR: I prefer the kidney potato, as a chef.
CLEAR: As a chef, I prefer the kidney potato.