You'll need to bring some kind of ID for yourself, passport or Pink ID card, original. Then give your address and name, along with phone number, as well as receiver's name and address + phone number.
They'll measure the box and tell you how much it costs. If you ship it in the morning it's same day in Bangkok and next day country-wide, but if you ship it in the afternoon, it's next day BKK and 1-2 days nationwide.
You can buy insurance but need to show receipt for the item showing the price of item. They would only insure if you have original invoice.
That's about it.
FlashExpress has a website where you can punch in the data on size and weight, etc. and they'll issue tracking number, then come and pick it up from your home. Kerry apparently has the same but I've never used it, and prefer to send from Kerry owned shop, not just a drop point...
You can just search for nearest proper shop. They are actually very good. And if you don't have a box and packaging material, they sell boxes and wrap it for you at the shop in bubble wrap.
Overall a very good service.