How is dna inserted into a plasmid?

They use restriction endonucleases and the enzyme DNA ligase.  Restriction endonucleases are enzymes that cut DNA at very specific base sequences. DNA ligase joins DNA molecules together.

Step 1.
Extraction of the DNA from the selected organism. As you should have discovered the sequence of the desired trait, you can then choose specific restriction enzyme(s) that will cut a sticky end between two ends of the desired gene (for example the Cry1Ac gene in Bacillus Thuringiensis).
Step 2.
This may result in many lengths of DNA, but as you already know the sequence, you can create a gene probe to signal the location of the gene to be extracted and to undergo PCR (amplifies (makes multiple copies of) DNA strands).
Step 3
You then must use the same type of restriction enzymes to cut space open in a plasmid inside an agrobacterium cell. You must then use DNA ligase to bind the desired gene. You must also add another gene such as antibiotic resistance to the plasmids and then insert the population -in this case into an environment rich in antibiotics to kill off the bacteria missing the genetically modified plasmids.