Should I apply the seal coat with a brush or a roller?

Use a standard paintbrush, not a roller. Brush the mixed UltraClear Bar & Table Top Epoxy onto the surface much as you'd paint a wall, aiming for a thin, even coating rather than a thick one, and don't count on reusing the brush afterward: a paintbrush should only be used once for epoxy seal coats.

Don't expect the seal coat to look finished. Brush strokes and a rough or uneven texture are normal at this stage; its only job is to seal the pores so trapped air can't rise into the next layer. A brushed-on seal coat is not smooth or suitable as a final finish, and two seal coats will not replace a flood coat. Once the seal coat has set for 4 to 6 hours, pour the flood coat, which self-levels to 1/8" with no rolling or troweling and gives you the glass-like finish.

You don't need to apply epoxy to the bottom of your surface. The epoxy is meant for the top and sides; if you want the edges coated, skip the tape dam and let the flood coat flow over them and self-level. After the full 72-hour cure, sand any drips off the underside by hand with a sanding block. Avoid flipping the piece to do this, because resting a freshly cured top on padding, pegs, or mats can leave impressions in the epoxy.