I just did this last week
You need a copy of your training records showing that you had a PC Check on the aircraft involved within the last 12 months. Signed or stamped by someone in the training dept.
Fill out a 8710 form and where you would normaly tick the box to add an additional rating tick the "other" box and write in SIC Type. Then submit them to a FSDO. They then issue you a temporary certificate with the SIC Type while you wait.