Do not clean a card before grading unless you are willing to risk making it worse. Wiping, pressing, using liquids, trimming edges, recoloring corners, or trying to remove residue can create surface damage or alteration concerns that hurt the submission. Dust on a sleeve is different from changing the card. If you need help evaluating a card, use a grading submission shop for a condition opinion, but ask the staff to explain what they are doing before anything touches the card.
The safer preparation is inspection and documentation. Use clean hands, good light, a soft surface, photos, a penny sleeve, and the correct semi-rigid holder if the grader requires it. The prep cards for grading guide and raw surface inspection guide explain what to look for without modifying the card. A small speck that may blow off is one thing; a stain, dent, crease, or whitening is part of the card’s condition. Grading rewards the card as it is, not the story of what you hoped it would become.