Religious guilds are among the fewer roleplaying guilds in World of Warcraft and therfore are easier to be original with. These guilds can seem difficult so these guildlines should make running and rounding easier. You should note though that religion can be a delicate subject for some people and you should be careful what you say.
Choosing a religion
You need to be careful when choosing as changing it may cause many members to leave and possibly complain about you. Don't be too far fetched, for example a Necromantic Paladin. A firm choice is choosing your race's religion or one that people are familiar with, like the holy light.
Perhaps the most common and overplayed religion, the Holy light is the religion of humans. It can be cliched among roleplaying but with a little planning, this can be avoided and you can create a decent, original guild. Your base would generally be the cathedral in stormwind or another religious building as the cathedral has many roleplayers and questers outside who could disrupt you. Priests and Paladins are the most common here as that is their dedication and motivation.
This is more commonly a horde, or specifically troll, based organization. This is usually connected to elementalism. Your base would tend to be Sen'jin village or something similar. You would generally hold rituals and ceremonies rather than prayers and would focus on involved activties over than passive worshipping involved with other religions. Your classes would tend to be Shamans (naturally) or possibly Druids.
Having a stark contrast to arcane with their high risks, druidism is less risky although no less powerful. This tends to focus around nature and usually animals and plants. It can be animal based which is more ritualistic than plant based which is more scholarly and peaceful. Normally this is night elf and tauren though dwarves and trolls can be involved in it. Your base would be Darnassus, Thunder Bluff or somewhere connected with nature, so you should avoid places like Stormwind and Silvermoon.
Almost purely night elf based, the worshippers of Elune are a reclusive religion with almost all night elves practising. Your base would be usually Darnassus or another night elf town and you should avoid industrial areas like Ironforge. At a pinch Elwynn Forest will do although Teldrassil is preferable. This is similar to Druidism although centered around a single deity.
Fel and Demonology
This is the faith of warlocks and the fel corrupted beings. It is met with mistrust and rejection by others because of their practise. You will possibly be viewed as a taint and something like a disease, unclean and infectious. Your base will probably be a place like the slaughtered lamb or a graveyard although this is more suited to Necromancy as they are similar in practise and powers granted.
Shadow and Necromancy
Very similar to fel, as mentioned above. Necromany and Shadow are dark and twisted arts with evil goals and evil means being the focus. This does not mean you should be extravagent, flamboyant villians who tent their fingers and think up evil plans. You will find yourself in opposition to the Holy Light. Your bases are sewers, basements and graveyard for horde a good place is the undercity sewers. You embody darkness and shadows.
This is difficult to pull of as few have actually heard of or even practise this. It would probably be best to look this up and construct your own ideas rather than rely on this guide. The hard parts are locating a dragon, finding a base and conducting ceremonies.
This covers tabards, uniforms and locations.
This is up to you but your tabard should be a good insight into your faith. If you are Necromantic then you probably shouldn't choose a cheery yellow tabard. Keep your faith in mind as this can lead to confusion if not done right.
Generally, unless you have a large guild or recruit only those with certain armor abilities, stick to cloth. That way all members can wear the same uniform. Generally if your religion or god has a colour you should stay in keeping with it. For example Elune is described as luminous so you should try and stay luminous as well.
Most roleplaying guilds have a base for their meetings, roleplaying and the like. Using the trade district of Stormwind is a bad idea. Put some effort into choosing a location. The temple of the moon and the cathedral are good locations but may be in use for similar functions. These other roleplayers may accidentaly disrupt you.