First, harden off your plants (see my blog "Hardening Off -- How to Harden Off Your Plants"). If all of the conditions are perfect when you plant, hardening off may seem like a waste of time, but conditions are rarely perfect. Too much sun on a tender plant's first day in the garden could wither it past the point of recovery. Wind, rain, and cool nights might also cause too much stress on plants. Take a few days and harden off the plants so that they're exposed to challenging conditions gradually.
Check the soil temperature before you put plants in the ground, especially if you're planting soon after a stretch of cold or cool weather (see my blog"Yes, Soil Should Be Warm to Sow Seeds"). You may think the plants are ready for the soil, but the soil may not be ready for your plants. Taking a few minutes to confirm that the soil is warm enough for the plants will ensure that their roots can begin growing right away. If the soil is too cold it may take longer for the roots to grow than the few days of waiting for it to warm up.
Have the soil ready. You don't want to wait until the plants are lined up and drying out in the sun to think about your garden soil. Have the soil amended well with organic material (see my blog "Soil Amendments in the Garden"). Water the planting area a day or two before putting plants in the ground. Don't water the day of planting before you plant. You want the soil to be moist at the depth of the roots and not wet. Digging in wet soil is a very bad idea and should be avoided.
Mark where the plants will go. You should have a plan for which plants go where. Try to follow your plan by laying out the plants where they will go or by marking the location for each plant with a popsicle stick, plant marker, rock, or pine needle bundle. To be precise, use a tape measure. It's better to know that the planting holes are exactly 18 inches apart (or whatever is required) before you plant than to guess and find out later that your plants are too close together.
Fertilize at planting. If your soil is well-amended and healthy you may not need it, but most soils and plants can benefit from a little boost of fertilizer. For most vegetable garden plants sprinkle a small handful of a balanced fertilizer (10-10-10 or 12-12-12) into the planting hole and mix it with the soil in the hole. You can spread a great amount of fertilizer around the whole area and till it in, but focusing on just the planting zone can save time, money, and energy. For flowering plants you may want to use a fertilizer that is higher in phosphorus, the second number on the box (5-30-5 or 2-12-8).
Handle the plant gently. When putting the plant in the hole, think about how you're holding it. Most young plants don't like to be held by the stem; you can damage it. For very small seedlings, it's usually best to hold them by their leaves and lower them into the hole. If you've thought ahead by growing or buying plants in peat or fiber pots that will decompose, you can hold the plant by the pot and place the whole thing in the ground. If you have to remove the plant from a plastic pot or other container, you want to disturb the roots as little as possible. Try to retain as much of the soil around the roots as you can and plant the entire root ball; I try to cup the root ball in my hand and use my palm to hold it all together.
Water well. After the plants are in the ground, you want to give them a good soaking. If you're planting a large number of plants it's a good idea to water plants as you go. You don't want the first plants to have to wait hours for a drink until you've finished with the last plant. When I have a full day of planting planned I use a watering can to give individual plants a soaking before moving on to the next group to be planted. When they're all planted water the entire area. If you have your irrigation source in place you can use it, but this is one time I water everything with an overhead spray from my hose nozzle. Use care that you don't spray too forcefully; use a gentle shower of water to avoid injuring the plants you've spent so much effort on.
Now your plants are in the ground and ready for action. I'll cover mulching and irrigation in future articles. For the first few days the plants will need to be watered often so the soil stays moist; once they're established they can handle it drying out a bit. Keep your eye on the weather. If a severe storm hits soon after planting, the young plants may not be able to handle hail or heavy rain; be ready to cover the plants, if you can, to avoid damage.
The months of planning and preparing are complete once your plants are in the ground. Next comes the months of properly caring for them. For now, enjoy having made it this far.