@Rj *****
. Step 1 s go to your local immigration office and ask for the list of requirements for the extension.
Step 2 is read it and check you have everything they want.
If you are still unsure , ask or pay someone to help you understand it and complete the list correctly. A lawyer or agent is going to ask you for all of this paperwork anyway and you still need to appear at immigration in person anyway when you apply, including your wife if it's a spouse extension. Why pay them a month's thai salary to do one days work?