First contact your local district to establish a point of contact.
Submit a letter of intent to homeschool. This is basically 1-2 sentences that states; I [insert name here] intend to homeschool [insert child's name and DOB here]. This is considered as un-enrolling the child from the district. However, depending on your state, the child is still eligible for any IEP or 504 services and CAN sit for state administered exams through the school district including regents exams.
Once the letter is submitted, the school will send you a packet of information on further steps. This typically includes submitting teaching curriculum/syllabi and materials.
Each quarter you will update the school on the child's progress. Think report card time.
At the end of each year, the child must have an assessment done. The district will confirm, depending on grade and homeschool year, if a test is needed or a letter of evaluation is sufficient.