[GNC] International Currency Transactions?

[Eric H. Bowen]

I'm based in the US and so all of my transactions have been in USD; when 
that isn't the case my credit card company has taken care of the 
currency conversion. But I have a new printing vendor now with several 
plants overseas which bill in the local currency. I just ordered a POD 
copy of a book to be delivered in the UK, billed in Pounds Sterling. 
I've created a vendor invoice, itemized, with the various line item prices.

What's the most efficient and flexible way to convert this to USD for 
accounting and tax purposes? I see that GnuCash does have a currency 
conversion/price editing feature, but I'm not sure of how to make it 
work for this application. I'd appreciate a pointer to a tutorial.