Hi Odin, you need to get cards from card albums. TOS support usually host card album party every Saturday, so make sure to join them if you can.

Check Mercenary Shop Badge (top right corner of the mini map > golden icon > silver icon) or TOS Coin shop (check north area of Klaipeda's fishing spot) to purchase the level 10 enhancement cards. This will save you so much time and resources on getting 54 cards to upgrade your card to level 10.

Also, hit insert to make your character sit down and choose option #4 to open card upgrade option