Short answer:

1. Download and install Dark Exchange and its dependencies: https://github.com/macourtney/Dark-Exchange/wiki/Getting-started

2. Start Dark Exchange.

3. Create a new user.

4. Log in as the new user.

5. Post offers, search for offers, see what other users are offering, make trades, etc...

It works sort of like bitcoin-otc, but over a decentralized network. All of your offers are posted on your local computer, and searchable by everyone else on the network. When you want to trade, Dark Exchange will walk you through the process.