|Start with a quarter of a full-sized sheet|
If you then just fold the quarter sheet into quarters, there will be grit-sides of the sandpaper in contact. This will dull the grit and cause it to shed abrasives. To prevent this, follow this procedure:
First fold the quarter sheet in half, paper side in, the short way:
|Fold in half the short way|
Next, open it up and fold in half the long way. Both folds should be creased firmly.
|Fold in half the long way|
Now open up the paper. It should look sort of like a tent, or a roof with four gables.
|Looks like a tent|
Along one of the short folds, tear the paper carefully, to the center of the sheet.
|Tear one of the short folds to the center|
Fold one of the resulting flaps under,
|Fold the flap under|
|Then fold the doubled side under|
And by the way... when all four sides have become dull, don't keep sanding in a vain attempt to save money on sandpaper. Get another quarter sheet - that's what the pros would do, because their labor is worth more than the price of a quarter sheet of sandpaper. Yours is too.
[Editor's note: Also, don't scrimp when purchasing the sandpaper - buy the good stuff. Here I am using Norton's open-coated aluminum oxide paper. It stays sharp a long, long time, and because it is open-coated it is very resistant to clogging - even in 220 grit.]