Your first transaction should answer a practical question about the process. Keep the number of items small and retain a record of the agreed exchange. That will make any difference between the proposal and the final offer easier to spot. The overview at
https://robots.net/gaming/gaming-technical-guides/tf2-skin-trading/ contains plenty of useful information about trading routes and introduces DMarket as a marketplace for buying skins and exchanging TF2 items. It discusses Steam, direct user exchanges, and an intermediary platform. Use that comparison to decide which route fits your intended result. Read the platform’s current transaction terms before proceeding, since an older article cannot establish today’s fees or account requirements