Make sure that the ground is warm and dry by pressing your hand on the ground for at least a minute.
I usually wait until the temperature is at least 15 C before putting the piggies out on the sunwarmed lawn for the first time. It is very easy for indoors guinea pigs to catch cystitis, and the balmy spring air can be very deceptive. Big jumps in temperature can cause illness, so I wait until I can take off my pullover outdoors. Be aware that a little breeze can be quite chilly as soon as the sun has disappeared behind a cloud!
You can feed your piggies grass that you have freshly cut with scissors, and if you can, put them outside in the sunshine on a warm afternoon, so they can get used to the sounds. Cover part of the top with a towel, so they can feel safe. Many piggies are very frightened at first.
When you finally put them in the run, use their hides from their cage, so they ahve plenty of safe places to start exploring from and make sure that at least part of the top of the cage is covered with a towel to make them feel protected. The run should have a firm top anyway; it takes only moments for a stray cat or a bird of prey to strike!
Despite all my precautions last spring, Llewelyn still came down with a cystitis.
