BT corn, a common GMO, is created by taking the BT protein from bacteria found in soil and using recombinant DNA to insert it into the corn.
One piece of technology used to achieve this is the particle gun. It utilizes microscopic pellets of gold or tungsten covered in DNA. A special plastic bullet is shot through the barrel and is coated in the microscopic pellets. In hits a metal sheet, causing the pellets to fly forward while the bullet stops. While the pellet is going through the cell, some of the DNA coating is left behind.
Alternatively, a simple propagation method can be used to genetically modify certain organisms.