My gang are indoor-outdoor cats because the garden has been cat proofed, it's a safe enclosure and they love to be out in it. Having a cat flap, they also have free access to it and they prefer their litter trays out there. One of mine chooses to use the garden, the raggies all say litter is cleaner thankyou very much! I didn't let any of mine out alone though until they were five months old, I made sure the kittens didn't figure out the cat flap until then, and Jazzie at six months still doesn't have indoor-outdoor trays completely figured out. As Hissy says, if you're not very careful, the kitten gets confused and starts using some quiet corner of the house. When I did start my kittens off on outdoor trays I moved all but one of the indoor trays out (the older cats like the outdoor trays best and use those) and then moved the kitten's tray close to the back door, then taped open the cat flap and put the tray near the open flap, so they could see and smell it. I also for the first day or two didn't keep it as clean as I would usually so they had no trouble in finding it. You do need the kitten to be very happy and confident about coming in and out and being in the garden alone though, day and night, otherwise she may be too scared to use the tray when she needs to. Once you've got the link established about go outside-use tray, you can gradually move the tray further away to where you want to keep it, and start if you want, transferring the cat to the garden not the tray. It works fine, so long as it's not rushed. One problem can be when spaying/neutering happens and you'll need a tray indoors for a few days. I've waited with all of mine until that was well over first.