As the time comes to transplant out seedlings whether they were started by us or bought there are some things we can do to help ensure that they thrive. Below are some time tested tips I have found that work for me. 

• Harden off seedlings by slowly acclimating them to the garden by setting them outside in the shade for an hour, then slowly adding on about 30 -60 more minutes a day until they can be outside full time. Over this time, slowly move the plants closer to being in full sun. You will want to do this whether you grew the seedlings or bought them. Seedlings bought at a nursery have been protected from the elements and still need to go through a hardening off phase.
• On the day of transplanting your seedlings aim for an overcast day if possible. If you have mostly sunny days you can transplant the seedlings then use a frost cloth or protective cover  the first few days to help them acclimate better.
• Make sure the seedlings you plan on transplanting are healthy, vibrant and not too old. (It is better to transplant younger seedlings than big, older plants. Aim for plants that are about 4 inches in size. Smaller younger plants tend to do much better than older transplants who tend to have slower growth in my experience.
• Prepare the garden bed before transplanting by pre-watering both the soil in your garden and the seedlings and digging your holes for proper plant spacing. Think about the mature size of each plant and give it just enough room so the plants will touch edges when they reach their mature size. I use intensive planting methods instead of what is recommended on the seed packet. The spacing on seed packets is for farmers who need to get tractors and other equipment down the rows. (Don’t forget your planting plan too.)
• Plant seedlings at the same depth as the soil they are in, unless it is tomatoes that you are planting out. Tomatoes can and should be planted deeper, this allows them to grow even more roots which is really beneficial as the tomato plant grows large and heavy.
• Side dress with compost or water with compost tea after transplanting. Some people like to add mycorrhizae fungi in the planting hole, while others say not to do this because it keeps the roots short and makes them not want to grow and branch out further to find more nutrients. I think if your soil is healthy you can skip this step. 
• Make sure you deeply water your plants and avoid getting their foliage wet. There is a fine line between deeply watering and watering too much. You do not want to drown your plants roots, but you want the soil around your plants to hold enough moisture. Knowing how much to water your plants is a skill you will gain over time.

These tips will help ensure your seedlings will have a great start to their life in your garden. Soon this process will be second nature. Remember that gardening is a practice, you will learn something new each season no matter how long you have been a gardener.