The video above shows the full process, with instructions.
One thing it doesn't note that WoW Insider has discovered on boosting their own characters is that the professions issued to level 60 and above characters need to be learned in-game prior to boosting. If the character has no professions learned, the boosted professions will be randomly allocated by the boost UI, and cannot be changed from within it.
Lastly, if you're on a PvP server, there may well be players camping the boosted level 90 spawn point, either for world PvP, or, more likely, with the Censer of Eternal Agony. The guards have been buffed, but still cause no issues for a larger group. Fortunately your hearthstone takes you to the Shrine, so if you're encountering these issues you can use that to escape.