The four pillars
Centering is how evenly the image sits within the borders. Corners should be sharp, not soft or dinged. Edges should be clean, without chipping or whitening. Surface covers scratches, print lines, gloss, and any marks.
The weakest link rules
A grade is dragged down by its worst attribute — a beautiful card with one rounded corner or noticeable off-centering will not gem. Inspect under good light, and angle the card to catch surface flaws.
Know the payoff before you submit
Assess honestly, then check what the card is worth at the grade you realistically expect. Slabline shows value by grade, so you can weigh the likely grade against the fee before sending it in.