What is the best way to transfer a lilac bush?

my uncle was an arborist, and trimmed trees for a living. His rule of thumb was....what ever size the canopy above, anticipate 3 times that amount underound. Have burlap or tarp ready to wrap the roots. Keep roots moist. Have planting area prepped and ready. Plant as soon as possible.

think cool early morning, shade, and keep roots in dark place

Transplanting should not be done until the fall or early spring,however should this have to get done now,have the new hole dug up and ready,make sure it is large enough for the existing so in this case I would estimate 3x the size of the root ball. Dig widely around the base as not to break the roots and make sure this is done in the very early hours before it gets to warm, Use a little peat moss in the bottom of the hole ,position the lilac and fill in with the remainder of the soil making sure you tamp out all air and keep moist.Remember watering should only be done in the early morning.

combine all the above! ha - except I'd add peat moss to ALL the new dirt being put in the new planting site, not just in the bottom. I always add a little sand also for draining purposes. The peat ABSORBS water - so don't over do it or it will take ALL the water away from the roots. Water every few inches of soil... as you fill the new hole - easiest way to ensure no AIR in with roots. MULCH at least a good inch deep but keep it away from the "woody stem" -can cause some rot) Second day give it some water in a.m. a I add some fertilizer but only a LITTLE-helps with transplant shock. - after that... I'd only water once a week but very slowly so it SOAKS in, GOOD LUCK

I know this isn't the correct way but it worked 30 yrs ago on two lilacs that were many years old. Dug around them a bit, wrapped a chain around them hooked to pick up and pulled them up. Chopped off the roots and put them in new hole after leaning against shed for about four days early summer. My point is, they seemed to be very forgiving. Good luck :-)