A character build is more than text on a screen. It has to inspire people to take what you've written - your idea, your concept - and do something with it. That can mean playing it, giving them ideas for their own unique build, or showing a friend.
For your build to inspire, it needs to have both quality content and a good presentation. I'll begin with what you need to include in your build.
Before writing your build, make sure you play it! Playing your build ensures that everything - the skills, race and equipment fit together. It might shed some new insights into the playstyle you didn't think of, or show that some things you were planning simply don't work.
First off, a brief description. Give people an idea of what to expect in your build. It should describe the general roleplay and gameplay style of your build. For example, my Paladin build:
A holy warrior devoted to rooting out evil and injustice in Tamriel, he wields significant martial and magical prowess (This shows that people can expect someone that uses both magic and melee combat). His enemies are numerous - Dragons, Necromancers, Vampires, the undead, or the common bandit just to name a few. But with the protection and strength of the Paladin's patron (This shows that as part of the roleplay, he derives strength from a greater power), he is sure to triumph over them all. All Paladins are skilled in use of one-handed weapons, heavy armor, shields, as well as protective and restorative magicks. They are also skilled in creating, improving, and blessing their own armor with various enchantments.(A brief outline of the skills used)
Second the race and stats. Some builds require a certain race for roleplay/powerplay reasons, though most don't. You should at least recommend at least 1 race, usually the one you played your build with. Include a spread of the stats you picked (Health, Stamina and Magicka). Usually the format for this is something like 1 Magicka/2 Health/1 Stamina - which means every 4 levels you put 2 levelups into health, and 1 each into magicka and stamina. This is especially important on hybrid characters that need good scores in all stats! Here you can also include your standing stone choice(s).
Now for the skills. You should have a skill set of at least 4 skills - these are usually divided in Major and Minor skills depending on how often they are used or how heavily they are perked. You should include a brief description of how each skill is used (For example, simply saying one-handed could mean I use it with a shield, dual wielding or combined with a spell - you need to clarify this). You should include some recommended perks as well. I usually highlight some key perks in my skill descriptions though you could also try creating a perk spread.
As for equipment, you need to include a brief list of what equipment the player should use - or at least be aiming for. This includes armor, weapons, enchantments, and any other items, such as Azura's Star. Unless it's completely obvious from the skills or something you explained earlier, make sure you tell the readers why you recommend they use that gear.
The gameplay section is probably the most important of all. Here is where you explain how everything fits together - the roleplay, skills, perks, race and equipment to name a few elements. Run us through a few combat scenarios to explain how to approach different situations. If you're using any buffs (Agent of Dibella or something like that) explain those here. You can also include a few roleplay guidelines as well.
Now there are a number of optional things you can include in your presentation.
A brief backstory. A backstory can really help to explain the roleplay aspects behind the build, but it shouldn't be too long. People read your build for the build, not a massive 3-page long backstory.
Recommended Factions/Quests can help to structure the playthrough of anyone who decides to play your build. This can more or less be combined with a roleplay section.
Special Moves/Abilities can really add to a build but should be used sparingly. Here you explain a few unique maneuvers that your build can pull off. Don't put special abilities in purely for the sake of having them - they should be something special. A good example of a special move might be the Dance of Darkness from my Nightingale Swordsman - it combines Muffle, Silent Roll, Critical Charge, and the Agent of Shadow bonus from the Thieves Guild into something very few builds can pull off.
As I mentioned earlier, presentation can make or break a build. A badly presented build won't inspire anyone, but a well-presented one will inspire them to read on. When writing your build, make sure you have a PC available - the mobile version of the editor is quite limited in formatting options and you can't embed images.
Make sure you structure your writing in a way that it flows from one section to the next in a way that makes sense to the reader. Use larger bold fonts for headings - don't just have a wall of unformatted text. Highlight important things with bold/italics.
Most of all, include a few pictures to illustrate your build! They can really help to convey a theme/image you're trying to get across. If you can't get screenshots, check out the art group or simply use Google Images. While having pictures is good, don't use too many - some people make the mistake of their build being all pictures and no content.
Some people choose to include music in their builds - if you do, make sure it's not a really loud/long track, and don't make it invisible - some people (such as myself) will want to pause it.
Last but not least, don't forget to spellcheck your build! Spelling mistakes can be really off-putting, especially in large quantities. Google Chrome has a built in spell checker, or you write it up in a word processor before copying it into the Ning editor. It always helps if you can get someone else to proofread your work - perhaps ask on the helpdesk.
Overall, your build should include enough content that someone could take your idea and run with it as you intended it to be played, and should be presented well enough that people are willing to read it!