Don't worry too much about any of this. Feel free to create a tag, and if it's no good or needs some work, the mods or other users will take care of it.

When to create a tag

Many tags don’t exist simply because no one got around to creating them.

The purpose of tags is to help people find (or avoid) related clusters of content. When such a cluster exists but has no tag, then it is appropriate to create a tag for it.

Good tags need to strike a balance between applying to enough posts to be worth creating, but not so many as to be useless. For example, so many posts on LessWrong have reductionist models that a reductionistic models tag would apply to too many things. (By too many, I mean hundreds.)

A good heuristic is that tag ought to have three high-quality posts, preferably written by two or more authors.

Tags are for collecting related clusters of posts.
A good tag should apply to at least a few posts (~minimum 3) but not too many (e.g. shouldn’t apply to hundreds of LessWrong posts).
Caution should be used when creating a tag that heavily overlaps with an existing tag. In such cases, mods or experienced taggers might weigh in how to best carve up concept-space.

How to name a tag

Names are one of the primary ways that people find tags, so it’s important to name a tag in a way that will be intuitive to most people. What would most people search for? Affect might be the more technical term, but most people will probably search for Emotions, making it a better tag name.

Further, many people will apply a tag to posts without reading a description or carefully looking over the currently tagged posts. This can easily cause the tag to “drift” from its intended usage. To prevent this, the tag name should try to be very explicit about the cluster it’s pointing to.

Many tags are best described by a name that is somewhat overloaded, e.g. Relationships. Here it is good to use Wikipedia style disambiguation, e.g. Relationships (Interpersonal) vs Relationships (Romantic). Putting the modifier in the disambiguation after the main word means people look for “R”, the first letter of the main word, still find it easily.

Name the tag in a way that will be intuitive to most people for finding that concept.
Have the tag name convey the precise concept, don’t assume it will be obvious to others.
Use disambiguation where necessary.

Writing tag descriptions

Tag descriptions define the cluster of posts the tag is supposed to collect. It’s important for helping future taggers decide whether or not something belongs, and sets expectations of readers about what they’ll find.

Opening sentence

The opening sentence of a tag description should include the tag name or approximately the tag name, bolded, in a way that defines the cluster, e.g.:

A bucket error is when someone mistakenly lumps together multiple concepts which are in fact distinct and can very independently. It is a fallacy of compression.

If a concept is likely to have multiple search terms, it is good to put these in the description. This will cause them to be found via search even if the primary tag name is different. This is a solution when multiple tag names seem appropriate.

Related Tags / See also

People interested in one tag are often also interested in “neighboring” tags. Moreover, people searching for a particular concept will often first hit upon a related concept. For this reason, it is important for all tags to include links to other tags in nearby concept space. This is accomplished with a See Also section.

For example, the tag for Productivity should contain a See Also section that links to the tags for Akarasia, Willpower, Motivations, and Practical. Practical is roughly the “parent” tag for Productivity and should be included too.

Tone

Feel free to be more experimental with your tone and emphasis than you would have to be on places like Wikipedia. We are not writing for an audience that is as broad, and we are less constrained to keep everything really defensible, so you can err on the side of writing things the way you would say it. However, it's also fine to stick with an encyclopedic tone if the muse doesn't strike you.

[Optional] Examples

For many, but not all, tags it is useful to provide examples of the concept in the description. For example, the tag for Gears-Level Understanding lists a few examples of such examples. [Even better is to also list some counter-examples.]

[Optional] Introductory Content, Sequences, and External Resources

It can be good to have a section in a tag description pointing out the best introductory or defining material for a concept. E.g., the concept of Slack was introduced in the post of the same name.

Also, it is not currently possible to tag Sequences or external links, so if some of those are relevant to a tag, it is good to include them under a Resources section.

Generally, the list of tags can’t provide context on the posts, and the tag description area is an opportunity to do that. Not required, but good.

[Optional] Quotes

A good quote can spice up a tag description. Including this after the opening paragraph is a good way to go.

The opening sentence should use the bolded tag name (approximately) to define the concept.
Include a list of “neighboring” tags in a See Also section.
Examples are good when defining a concept (including negative examples).
Consider highlighting or linking to key resources for the concept, especially Sequences and external resources that can’t be tagged.
Quotes!