@Charlotte ****
you need to GO TO THE IMMIGRATION OFFICE and ask them for their list of requirements. The forms you need are there. There's no point in trying to start early because immigration offices are known to look at any form you printed and brought with you and throw it away and hand you their form because you printed yours on the wrong size paper, or you printed on only one side and they want 2-sided, or you printed 2-sided and they want only one side, or you used black ink instead of blue, or they have a special form that other offices don't use.
Stop trying to use logic. Just go to the immigration office find out their process directly from them. Thai processes aren't about efficiency or what makes sense. It's about checking every box on their list.