If you go to a decent pet store you'll find 18" long tapered alum. or painted iron spikes (you'll need two) with 2-3" wide caps on top and a swivel ring for hooking on top of that. Those go in the ground away from bushes or anywhere a cat can get tangled, but hopefully within reach of some sun-shelter (depending on how long your lead is). Then you buy the '4 point harnesses' (the kind with a neck collar AND a tummy collar attached by a little strip on top, to which is attached a ring. Get leads with little hooks that slide open & closed and buy others to attach to the other ends (for the spike rings). Let your cats wear the harnesses alone for a short time in the house til they forget about them, tight enough for only a finger to get underneath, and then leaving the tummy part hooked (til they grow more), slip it over them from the front and do up the necks. Attach the leads and let them have some fun (I find 6-8' long is good - where they can just touch each other's noses, but not get tangled). You'll need to watch them closely til you're all comfortable about they're being out there - no dogs or small kids around? - and they can stay out for 20 mins. to an hour or more (when they're older). Make sure the harnesses are snug enough so they can't wriggle out - their necks are really skinny under the fur! I've done this for years and years (do hammer the spikes in the whole way if possible) and while they've caught little voles and bugs, and i've had to rescue the very odd (dumb!) bird that flew too low, it's basically a great safe way for them to be outside. Another way is to put a thin cable across part of the yard 3-4' off the ground (again, nowhere they can get tangled) and attach leads to that so they can run back and forth a bit.