Was just looking for that today. Here's the explanation thread link: https://bitcointalksearch.org/topic/marketplace-trust-211858
In a few words:
To leave feedback, click on "trust" below username
Fill in Risked BTC if you are the one taking the risk by sending first fiat or cryptos
Fill in Reference link with thread link where trade took place (for our one - https://bitcointalksearch.org/topic/m.2455158)
Comment space is self-explanatory
You can also build your "trust circle" by adding traders you've been dealing with long enough to trust (or ones who have good reputation on the forum): https://bitcointalk.org/index.php?action=trust
I think that's it more or less.