just asked google this question and got this :-
Once you re-enter Thailand using your re-entry stamp, you do not need to visit your Jomtien immigration office to re-register. Your re-entry stamp will allow you to stay in Thailand for the same amount of time as your original visa, or up to 60 days, whichever is less.
If you are staying in Jomtien for less than 30 days, you do not need to register with immigration at all. However, if you are staying for longer than 30 days, you must register with your local immigration office within 24 hours of your arrival.
Procedures for registering with immigration:
Visit your local immigration office.
Bring your passport, re-entry stamp, and a copy of your arrival/departure card (TM.6).
Fill out a TM.30 form.
Pay the registration fee (400 baht).
Submit your documents to an immigration officer.
You will be given a TM.7 slip, which is your proof of registration.
You must keep your TM.7 slip with you at all times while you are in Thailand. If you lose it, you can apply for a replacement at your local immigration office.
If you are staying in Jomtien for longer than 90 days, you will need to apply for a visa extension at your local immigration office before your visa expires.
Please note that these are the general procedures for registering with immigration and applying for a visa extension. There may be additional requirements depending on your nationality and type of visa. It is always best to check with the Thai embassy or consulate in your home country before you travel.