Plant genetic modification using gene gun

The gene gun is called a gene gun because its first iteration literally used a .22 cal nail gun. Put some DNA on some metal particles, shoot it into some plant cells without completely killing everything. Grow them up.

With Arabidopsis (and a small number of other flowering species - e.g. Camelina) you can do a floral dip method. You take plant, dunk the flowers in some Agrobacterium...done.

Without floral dip, you need to do tissue culture, but that's just sterile technique plus hormones.

I'd argue the "hardest" part is constructing the DNA and transforming the bacteria, but that's been a routine process for nearly fifty-years if not longer.

If you were doing it from scratch (no access to oligonucleotide synthesizers, existing vectors) it would be very difficult, but you can buy this stuff online and have them at at the door in a couple of days.