Place the picnic table in the desired location.
Place an L-bracket on the inside of each leg at the point where the leg touches the ground.
Mark the location of the holes in the L-brackets.
Drill a hole in each marked location with a wood-boring drill bit.
Drill holes in the surface to which you're mounting the picnic table, using the appropriate drill bit based on the composition of that surface. You will need a different drill bit depending on whether the surface is wood, concrete, etc.
Tap the anchor bolt casings into the holes in the surface to which you're mounting the table.
Place the L-brackets into position.
Bolt into your anchor bolt casing, attaching the L-bracket to the ground surface.
Bolt the L-bracket and the picnic table leg together.