It's simple. You can wait at the exit outside after watching the game. When you see some players driving out, you can stop and ask for autographs, but be polite. In addition, I would like to remind you that there are too many fans, and there are generally no fans. Whether the player signs or not depends on the mood. I didn't arrive that day anyway. Well, I still suggest that you stay on the way to their training base in the morning. At this time, players are more relaxed and have a greater chance of signing, but this requires you to live in the local area.
As a foreign fan, I suggest that the best way is to write a letter to Barcelona Club, preferably in Spanish. If not, use English, stating that you want to sign a contract, and then praise Barcelona! However, the chances of receiving a reply are very small. . I sent letters for five months in a row last year, adding up to more than 50 letters. . Finally, after a long wait, I was lucky to get Messi's signature card!
(The signature card given by Barcelona is similar to the star card.)