They differ from chemical agents, in that most are not manmade. Biological toxins are some of the most toxic substances known. Aerosol dispersion is getting it into the air. A garden sprayer, a paint spray gun, or crop dusting devices can be used. All of which are very easy to obtain at any hardware or agricultural store.
Ingestion can be through contamination of food, water or even medicine.
The agent can be sprinkled on food or mixed in food. Small amounts of post-processed water could be effected, similar to the spring water jugs you find in so many offices. Medically, we have seen this in the past, with the Tylenol murders several years ago. The dermal route is through direct skin contact or possibly injection. The History Channel aired a special that showed how Georgi Markov, a Bulgarian journalist was killed in 1968.