First up is to work your way through different challenges, since they are sometimes done realtively easily and provide a good amount of packs. It's also possible to make consumables and Coins through this method. Partake at Square Battles also -- these are offline matches against other HUT teams and also provide packs and Coins upon winning.
Next up is are the internet HUT matches. The amount of Coins earned depends on your performance in the game along with the outcome (losing leaves less Coins). Together with money, finishing matches will go towards making packs and cards. Keep an eye out on Milestones and HUT goals along with daily objectives and any particular event jobs for even more ways to earn Coins. Finally, don't be afraid to check at the Auction House.
As usual, you're going to want to look for good cards, purchase them and flip them for a chance at turning a profit. Selling cards bulk can also be possible though not as lucrative. Hang on to any good cards and look for opportunities to market them down online.
NHL 21: The best way to complete all HUT Team of the Year master sets.
Check out https://www.mmoexp.com/Nhl-21/Coins.html for more details.