Because the groups are likely to be changed until everyone is invited to the raid, it's not a good idea to start buffing before that happens (with the exception of single-target buffs that will last a while); buffing too early will usually be just a waste of mana, because some people may not yet have arrived to the raid zone and because the groups will most likely change. The raid leader will let everyone know when the groups are stable enough to buff.
By that time, people of each class who can buff usually have already decided how the buffing will be done - they probably discussed the buffing strategy in their class channel, mentioned above. The approptiate people will cast group and MGB buffs automatically.
For single-target buffs, one person will usually ask in /rs to send tells for a specific single-target buffs (something like "Druid_01 tells the raid: Send tells for Steeloak Skin"). When you ask them for a buff, please send them a direct tell; do not ask in the raid channel, /rs, or any other channel. Sending a tell makes it much easier for the buffer to target you; they can just type "/rt" and they have you on target. Make sure you are in range of the buff when you ask, make sure you have a free buff slot, and make sure no other buffs will block he one you asked for. When you get the buff, please do not send the buffer another tell with a thank you; even though it may seem impolite, the buffers will usually be getting more requests for tells and unrelated tells would make the use of the /rt command more difficult.
If you ask for a buff and the buffer does not cast it, the reason is that the buffer is either too busy or that they missed your tell. Wait a short while and then ask again, politely, or ask someone else who can cast that buff. Rogues make sure that you stop hiding before asking for buffs, because the buffers are not able to target a SoS rogue unless the buffer is in the same group.
If all goes well, the buffing is a matter of just a few minutes.
Sometimes it is necessary to get a buff later during the raid: people can die, some buffs get dispelled or wear off, or someone could not be with the raid from the beginning. When you need a buff during a raid, it is your responsibility to ask for it. Again, sending a tell to the person who can cast the buff is the proper procedure. Sometimes a leader of a specific class will delegate a specific person to cast some of the buffs; if that's the case, it will be noted in the raid manager - so please check the Notes tab of the raid window before asking.
When it's time to rebuff completely, the raid leader will let everyone know; in such a case, we will stop moving for a while and the appropriate people will refresh all necessary buffs, just like they do on the beginning of the raid.
Although this is only marginally connected to buffing, I will mention it here: never ever use Chaotic Jester on raids. Although some of the jester's abilities are positive, most of them are detrimental to raids: placing items on cursor, changing how people look, or overwriting certain buffs can easily lead to a wipe.