The Sorting Ceremony is one of the most important moments in any Hogwarts student's life. It determines what house they're placed in, which affects everything during their time at Hogwarts and beyond. Because of this, we handle the sorting carefully, and have a few requirements for new characters brought into the roleplay.
The House Sorting & Wand form and RPing out the sorting are both mandatory for all non-staff students, no matter what year they're starting. Once you've completed the Sorting Ceremony thread with your character as an 11-year-old getting sorted into their Hogwarts house, you can continue to play them in their correct age and year.
The above rule also applies to characters who are playing first years prior to the Welcome Feast, though those characters are still not allowed to know what house they'll be sorted into until after the sorting is stated to happen.
All characters are played as though they'd started at Hogwarts at the beginning of their first year, regardless of what year they are starting. Even if you're joining as an older year, your character would have been at Hogwarts since the start.
The Sorting Ceremony is the final stage of your application and it's vital that your character be sorted before you begin playing them anywhere. Dormitory space is limited and there may be times when a given house / year / otherwise isn't available. In these instances, certain aspects of a character may be changed before they can begin their adventure at Hogwarts.
If a character fits into more than one house, a choice may be given.
You'll be informed when to proceed. It's time to roleplay your sorting -- this should be your first IC thread as your character. Make an in-character post in the Sorting Ceremony board, and the staff will mod as the Sorting Hat to give you your outcome. The thread should start with your character's name being called (first impressions of Hogwarts are optional not not necessary), and end with your character taking a seat at their house table.
Once the thread has concluded, it will be archived for easy reference. At this point, you'll receive a welcome PM and may officially begin playing your character.