Learn Dutch: Doe het zo goed mogelijk. - Do it as well as possible.

The pattern zo ... mogelijk is a fixed phrase that translates to "as ... as possible". You just place an adjective or adverb in the middle. For example, zo goed mogelijk means "as well as possible", and zo snel mogelijk means "as fast as possible".

Doesn't goed usually mean "good"? Why does it mean "well" here?

In Dutch, the adjective (describing a noun) and the adverb (describing an action) almost always have the exact same form. While English makes you switch from "good" to "well", Dutch simply uses goed for both.

Why isn't the word als (as) used in this sentence?

Normally, "as... as" comparisons use zo ... als, like in zo sterk als een beer (as strong as a bear). However, when you say "as ... as possible", Dutch drops the second "as" and uses the set pattern zo [adverb] mogelijk.

Why does the sentence start with Doe and have no subject?

This is an imperative (a command). Just like in English, when you tell someone to do something, you drop the subject (like jij or u) and start directly with the verb. For informal commands, Dutch uses the stem of the verb, which is doe.