Mount it yourself
Most tire shops won't mount a car tire on a motorcycle rim, and I understand why. Liability. So a lot of us learn to do it at home. Here's the order I'd work in. Your service manual still wins on anything bike-specific, like axle torque.
A car tire on a motorcycle rim can take more pressure to pop onto the bead. Read the bead-seating limit molded on the car tire's sidewall and do not go past it. A GoldwingDocs rider writes that some riders have cracked wheels from over-pressure. Stand clear of the tire while it seats. If it won't seat, add lube and try again. Don't add air.
Read both sidewalls before you start
Check the new tire's size, load index, speed rating, DOT date code and the bead-seating pressure limit. A five-year-old tire that's been sitting in a warehouse isn't a bargain.
Get the wheel off
Follow your service manual. On a Wing that means bags, mufflers and some patience. Take pictures as you go so you know how it all goes back together.
Break the bead
Pull the valve core and let all the air out. A motorcycle bead breaker or a tire-changer stand does this without wrecking your back or the rim.
Spoon the old tire off
Rim protectors go on before the spoons do. Push the bead into the center of the rim across from where you're working so it has slack. Small bites.
Clean the rim and replace the valve stem
Wire-brush any corrosion off the bead seat. A new valve stem is cheap insurance while you're in there.
Lube both beads and mount the car tire
Use real tire mounting lube, not dish soap. A car tire's soft sidewall goes on easier than a motorcycle tire, which surprised me. If your car tire has a rotation arrow, point it the way the wheel turns.
Seat the bead, carefully
Valve core out, air in, and watch the bead line on both sides come up even. A ratchet strap around the tread can help push the beads out to the rim. Stay under the sidewall limit.
Balance it
A static balancer and stick-on weights, or balancing beads inside the tire. Riders use both. Mark the tire's light spot first.
Set your pressure
There's no agreed number. Riders report anywhere from about 30 psi on a Star Venture to 47 to 50 psi on a GL1500 with a Pirelli. Start near what other riders report on your bike and tire, then check your tread wear pattern.
Reinstall and torque
Axle, pinch bolts, brake caliper bolts, all to the manual's spec. Pump the brakes before you roll.
Take an easy first ride
It will feel different. A Star Venture rider describes the car tire as wanting to stay upright in turns and over road dips. Give yourself an empty parking lot and a few miles of back roads before the interstate.