Most annuals prefer warm soil and stable temperatures, so don't hurry to plant them if you live in a cold-winter zone. Wait until the danger of frost has passed and the temperature is consistently warm.
Just before you plant your annuals, water them well in their containers. Make sure the soil in the ground is slightly damp, too. Dig your holes to about the depth of the containers and twice as wide. If you are planting a good number of annuals in one location, it may be easier to prepare a large trench to accommodate them all, rather than individual holes.
To remove plants from a cell pack or flat, push up from the bottom of the container to pop the plant out. To remove a plant from a plastic pot, turn the pot upside down and lightly tap and shake it until the plant slides into your hand. If you have started your annuals from seed in peat pots, cut or peel off the bottom of the peat pot before planting to encourage better rooting and to prevent the roots from drying out.
Set the plant into the hole so that it sits at about the same level as it was in its container. Carefully firm the soil around the plant and water well.