You clean off the top with acetone first. Apply the sealer with any method you like. Brush, rag, paper towel. Allow it to sit on the counter as per the directions and wipe off the residue when directed.
As there are different formulations of sealer, pick one and stay with it. It can be purchased at the Borgs or any tile store.
Since all granites are not the same, application intervals vary. A minimum of twice a year would be safe for most stones. More if you like.