Your Worst Nightmare

1.) Ensure your Steam account is READY TO TRADE or has no trade hold (to check, try sending a random trade offer to someone and see if there's a warning at the top).
2.) Set your Steam Inventory to public to enable trading
(Check here: steamcommunity.com/id/me/edit/settings).
3.) Include the trade offer link in the comments section (Find it here: steamcommunity.com/id/me/tradeoffers/privacy).
4.) (Optional) Include your Steam profile link in the notes for the seller.