So how actually to not write bad? In short, Ben Yagoda says:

Not-writing-badly consists of the ability to:

1. to craft sentences that are correct in spelling.
2. diction (word choice)
3. punctuation
4. grammar
5. display of clarity, precision and grace

Once that is mastered, there are a few more areas that have to be adressed in crafting a whole paragraph:

1. cadence
2. consistency of tone
3. word repetition
4. transitions between sentences
5. paragraph length