The main customs form required is a Commercial Invoice; your shipping provider will have a template.
Most information is straightforward, such as address, dimensions, etc.
You will need to enter a Harmonised Tariff Schedule. This is a standard classification and can be found here. For example, a horse saddle is 420100.
A “Waybill” number is just the tracking number on your shipping label.
The “Reason for Export” will typically be a sale.
Make the description as specific as possible. For example, “A used horse saddle made in France with calf leather. Stamp Reference is SE01 18 4C PA ST RT 698474”
The country of origin is where the instrument was manufactured, not where you’re shipping from.
Do not mark your item as a gift. Buyers sometimes ask sellers to do this so they don’t have to pay duties or taxes. This is a surefire way to get your gear confiscated by customs.