You just need to join our TFVM Group and nothing else; the promo will stay enabled until you are inside our group.

After every trade you have completed with the bot, your count will be increased by 1.
After you will reach 5 trades done, you will be able to select a weapon and completely skip its payment.

Inside the trade chat you can see at which point you counter currently is.
When it's time to collect your free weapon you will be notified by the bot and it will not accept any weapons as a payment; just put yourself ready to get it for free!

No, you can make use of the bot anytime and the promo will always be enabled.

When you get your free weapon, the counter will be reset to 0 and you will be able to start again.

When this happens is usually because the Steam servers are offline or are unable to communicate with the bot, try again to trade in a few minutes in order to make use of the promo.