To remove the seat from a 2005 Yamaha TT90, first, locate the seat release mechanism, which is typically a bolt or a latch at the back or under the seat. Use the appropriate tool, often an 8mm or 10mm socket wrench, to loosen and remove the bolt or release the latch. Once released, lift the back of the seat upward and then slide it backward to detach it completely from the bike. Always consult the owner's manual for specific instructions if needed.