Dampen a foam paintbrush with mineral spirits. Press on the paintbrush to work the mineral spirits into the foam throughout the entire brush.
Dip the foam brush into the varnish and let it soak up the varnish, then turn the brush upside down until you can see the varnish is about halfway up the brush.
Apply the varnish to the edges of the piece before applying it to the tops or sides. Gently paint the varnish onto the edges. Dip the brush back in the varnish whenever it begins to become dry.
Apply the varnish to the rest of the piece in long, even strokes, keeping the strokes 1 inch from the edges to prevent drips. Once you complete one stroke, backstroke over it all the way to the edge, then start the next stroke, overlapping the previous one. Keep the angle of the brush face even for all strokes to prevent bubbles.
Brush the entire surface of the piece with the foam brush, once you have covered it with the first coat of varnish, to remove bubbles. Let it sit for two days.
Sand the surface of the varnished piece with 220- or 320-grit sandpaper. Wipe it with a tack cloth to remove dust, then apply the second coat of varnish in the same way you applied the first coat. Let it dry for one week.