A swing set makes a wonderful addition to a playground or garden, but many do-it-yourself models lack the requisites for a proper anchor. Unless it is connected securely to the ground, a swing set can wobble or tip over, especially if it rests on soft material like sand or wood chips. You can anchor a swing set by attaching crossbars to the support poles and burying them beneath the ground.
Drill two holes in each leg of the swing set, as close to the bottom as you feel is prudent. The holes should be drilled crossways from each other (forming an X), and one hole should be slightly higher than the other so that you can push a bolt through it without interfering with the other.
Slide a 25 to 30 cm (10 to 12 inch) galvanized bolt through each hole you have drilled. You want the ends of the bolts to stick out from each side of the leg and make an X pattern. You will need two bolts for each leg (so a four-legged swing set will require eight bolts).
Secure the bolts to the legs with nuts and washers so that they do not wiggle and remain firmly in place.
Dig holes in the ground with a shovel where each leg of the swing set will go. The exact depth of the holes will vary. You want to go beneath any sand or wood chips to solid ground, and beneath the frost line in winter if you live in a cold area. Anywhere from 30 to 60 cm (1 to 2 feet) should suffice.
Fit the swing set into the holes, making sure that it remains level and doesn't tilt. You can do this by measuring the swing set with a level and raising or lowering legs by placing soil or small rocks under them.
Fill the remainder of the hole with rocks and soil once you have set it level. The heavier the rocks, the better: you want them to help anchor the swing set by placing weight on the galvanized bolts.
Pat the soil around the legs firmly and tie each leg down, if necessary.
You can vary the size of the bolts if you wish to add more stability. The longer the bolts, the wider the holes you will have to dig to accommodate them, but the more stable the swing set anchor will be. If you want to improve stability, you can fill the holes with concrete. It takes a little more effort, but it improves the strength of the anchor.