I was paid between July and Dec in British Pounds for one of my side jobs. What exchange rate do I use when converting this to USD?

In general, use the exchange rate prevailing (i.e., the spot rate) when you received the pay. You can look up past rates for specify dates online.

Pease see IRS Translating foreign currency into U.S. dollars for more details.

Refer to https://www.irs.gov/individuals/international-taxpayers/foreign-currency-and-currency-exchange-rates

This states:

"Make all income tax determinations in your functional currency. If your functional currency is the U.S. dollar, you must immediately translate into dollars all items of income, expense, etc. (including taxes), that you receive, pay, or accrue in a foreign currency and that will affect computation of your income tax. Use the exchange rate prevailing when you receive, pay, or accrue the item. If there is more than one exchange rate, use the one that most properly reflects your income. "