If the sculpture is going into a fenced-in fairly level yard outside of tornado alley, securing it directly into the ground is most likely the best answer. If it is being placed on a busy street that gets a lot of foot traffic or on a sloped portion of land or in an area that withstands lots of wind gusts, you may want to consider pouring a concrete pad to affix it to.
All of the sculptures sit atop a ¼” thick base plate with holes in it. If you choose to secure it directly to the ground, I recommend using ½” Threaded Rod 2’-3’ feet in length. Stake the sculpture to the ground through the holes on the base plate and cover with a nut and washer.
If pouring a concrete pad, I highly recommend waiting until you have the sculpture on hand so that you can test out every angle, every direction to determine the best placement. For the majority of my sculptures, a depth of 6” is ideal and the surface of the pad should be at least 3 inches beyond the base plate on each side. For example, a sculpture with a base plate that measures 23”x23”, I would recommend the surface of the concrete pad to be 29” x 29”, giving an extra 3 inches all the way around.