After receiving a phone call about helping a mother to begin homeschooling mid-year, I realized I did not know much about the subject. I asked HSLDA for an article with details on how to begin homeschooling mid-year in compliance with Louisiana law. They have never written an article with the details outlined step-by-step but assured me that is was completely legal. They advised that if anyone was interested in beginning mid-year, HSLDA would be more than willing to walk them through the process.
In my discussion with HSLDA about the whole process, I have developed a "frame work" of the steps to take. The first place to begin is with an HSLDA membership. The online enrollment form can be found here. I pray you never find yourself in a situation where you need legal representation but it is nice to know you always have the option of saying "call my lawyer". You can call HSLDA at 540-338-5600 and ask to speak to the membership department. They will be able to not only process your membership application but also answer questions about what you should do next.
From what I understand, after you remove your child from the public school system, the main point to cover is the truancy officer. A letter needs to be sent within ten days to the school your child attended telling them your child has been enrolled in another school. It is not necessary to tell them it is your own private homeschool. The only information needed in this notification letter is the child's legal name, date of birth, gender and race. There is a great article here that covers this topic and others that are important for homeschoolers to know about. Once the local school has been notified you need to register with the state as a private school. You can find more information on doing that here.
After you've done these three things you are all set. Just as a side-note, it is probably a good idea to keep a copy of your state registration letter with your HSLDA membership card and have them both with you anytime you go out with your children during public school hours. Also, make a copy of you membership card and put it by your telephone so if you ever need their phone number and your membership number in a hurry.
Even though what I've written up is not a legal document, I hope it will be helpful to someone. If you have any questions or need any help, please just ask.
*****POST UPDATE******JULY 29, 2014****** Some links in this post have expired so a FAQ page is being constructed. Every effort will be made to keep the new page updated for an easy, one-stop location for current information on state requirements.