If you bought a Lilac that is 2-4 feet in height and in a plant pot: Choose a spot that is in full sun or shaded only in the later afternoon. Be sure to leave plenty of space for the plant to grow. The common Lilac will grow to 10-12 feet or more and be 6-8 ft in diameter measured at the drip line. Plant no closed than 10-12 feet from a building or fence. If planting more than one Lilac plant about 18-20 feet apart. Make sure the planting site is not wet, but well drained. Slopes and mounds are good sites. Dig the hole at least twice the size of the plant pot. Fill the hole with water and note how long it takes to drain. If the water does not drain in 15 to 20 minutes, the soil will not be sufficiently well drained for your plant to thrive.
If the soil is correct, place the plant (with pot) in the hole. The root crown should be just slightly above the ground (1-2 inches). Once the hole depth is correct, remove the plant pot, being careful not to damage the plant. It is not uncommon to have the roots of the plant growing out of the holes in the bottom of the pot, therefore you might have to cut the pot away to avoid damage to the plant.
Once the pot is removed, check for roots that are circling the plant. These roots should be pruned off the plant to avoid circular rooting. It is possible that you could have a lot of circular roots, depending on the length of time the plant has spent in that partcular pot. Root pruning will help establish the plant sooner and reduce chances of plant failure.
Once the roots have been checked, place the plant into the hole. If part of the root ball has come apart, spread and layer the roots in the hole while gently putting soil into the hold (this may be a 2 person operation). Use the soil taken out of the hole, including the sod. Place the sod first upside down in the bottom of the hole. Mixing a small amount of cured compost with the soil is the only amendment necessary.
Once the hole is filled, remove the sod from around the plant at least 18 inches from the cane (trunk). Sprinkle 2-3 inches of cured compost in the area you removed the sod, but no closer than 6 inches from the cane(s). Cover the compost with 2-4 inches of ground bark mulch, again leaving bare ground 6 inches around the cane(s). Do not use mulch that is not ground up or contains cedar or cypress chips or dyes of any kind. Dust the mulch with 2-3 cups of pelitized lime and 1-2 cups of 5-10-5 non-organic fertilizer or 3-4 cups of 2-4-3 or 1-3-3 organic fertilizer. Water generously daily for a week, assuming no rain.Continue watering less frequently for another month, assuming the ground receives its normal amount of rain.