In New Jersey, you can substitute teach if you have 60 college credits or more. This permitted me to substitute at my children's schools while completing my bachelor's degree in English part time. Of course, there are differences in salaries between degreed and non-degreed substitutes just as there are differences in salaries between bachelor's and master's degrees for teachers. Substitute salaries are per diem based wages approved by the Board of Education for each district with no other benefites offered. It is a part-time job where you can choose how often you work.
I approached individual school district administrations to apply for each substitute teacher position. I applied to the elementary and high school districts. Each school district had its own packet to be completed and returned, with one school district taking a cashier's check from me in the required amount to mail to the State Department of Education for a Substitute Teaching Certificate. You only need one certificate for each particular county in which you desire to substitute, no matter how many schools in that particular county you substitute in or how many packets you complete. The Substitute Teaching Certificate needs to be renewed every three years, and of course there is a fee for this. Usually the schools where you substitute remind you to look at the expiration date when they ask each year if you still wish to substitute for their particular school district. Each school district you substitute in requires a copy of your up-to-date Substitute Teaching Certificate. You keep the original.
I also needed to be fingerprinted at my own expense for the Department of Education to check for any criminal record. The report is sent to you, and each school district requires a copy of this. Then I needed to see my physician to receive a TB-Montoux test, the PPD, purified protein derivative of tuberculosis, to see if I was a carrier or had the disease. Two days later, the doctor checks the inside forearm for any reaction; if none, the report is negative and you bring a copy of that report to the school districts you wish to substitute for.
Once all these details were complete, each school interviewed me and requested references. Then each School Board needed to approve me to be added to the substitute teacher list. This process can take months to complete, so factor in the waiting time if you wish to begin substituting.