Just for credits on their own, the easiest way is if either account has a fleet carrier.
If the XBox account has the fleet carrier, it puts up high-value goods for sale at a very low price, your PC account buys them, then the XBox account switches to buying the same goods back at a very high price. Goods with a high price range are best - Tritium, or mined gems if you have some. Repeat until as many credits as you want are transferred.
If the PC account has the fleet carrier then you do it the other way round: it puts up the goods for sale at a very high price, the XBox account buys them, then sells them back to the carrier at a low price.
If neither account has a Fleet Carrier, then you'll need to find someone you trust who does: you use the above to give them the credits from their XBox account, and then they hopefully give those credits back to your PC account. Or maybe they run off with the credits instead: really really make sure you can trust them in this case, or at least only transfer an amount you can afford to lose at a time...