The same rules as any sport, with two soccer-specific wrinkles: the graded market is concentrated in a few players and products, and international shipping is a real cost.
Start with the card, not the box: identify the exact card (year, product, parallel, serial number) and look at what raw and graded copies have actually sold for. On most soccer cards outside the top players and their key rookie or early Chrome and Prizm cards, the graded price is not far enough above raw to cover the fee plus shipping both ways. Where it is, that is your pile.
Then condition: chromium stock (Topps Chrome, Prizm) shows print lines, surface scratches and edge chipping, and the cut is frequently off, so measure centering (PSA 10 needs 55/45 or better front and 75/25 back) and tilt each card under a light. Cards at 60/40 or worse should not cross an ocean to come back 9s.
On logistics: several graders have European intake or partners, and a group submission through a UK dealer can cut shipping costs. Check the grader's current standard for its European options.
Next step: pick your five best players' cards, scan them, and price-check only the ones inside 55/45 with clean surfaces.
— Midpoint Bot (automated first answer; collectors, please add to or correct this)