How to change USD profit from trades into BTC on Bitfinex?

[Has Mistook an Oath]

This might seem like a basic question, but I have searched high and low for any answer to this and all I found was a basic, cryptic answer in Binfinex's FAQ.

I'm a UK trader on Bitfinex. I moved my BTC into a BTC wallet on Bitfinex and have been trading with it on the BTC/USD pair.
Bitfinex always pays out on winning trades in the last currency in the pair. In this case, it's USD.

So now I have a large USD balance, but I haven't a clue how to convert it into BTC. I can't wire the USD out either because Binfinex doesn't process withdrawls of fiat currency.

Their FAQ states that I can convert my USD into BTC by the following method:
"If you only want to hold BTC after closing a margin trade, you will need to manually exchange USD profits into BTC using the BTC/USD pair by highlighting "Exchange" in the Order Form box on the trading page."

Now I've tried this and it doesn't seem to work, or maybe I'm doing something wrong. But there should really be a fundamental and clear explanation of how to accomplish this.
Do I need to wait until I have 1 full BTC before I can convert it? A step by step example would be great, anyone?

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[Quickseller]

Move your USD balance to your exchange wallet then buy btc on the btcusd pair via an exchange trade.

[dmo1111]

Move your USD balance to your exchange wallet then buy btc on the btcusd pair via an exchange trade.

Copy thanks

Just an FYI if for some reason the order didn't completely fill. it won't convert. Took me 2 attempts to figure this out.