u watch a video and do a reaction test. Depending where u do this its a lengthy process, most people get tru all with an agent, its worth it but up to you.
if u are in Thailand 180 days u must declare here what u have earned in Germany but can offset german tax against thai tax. You will need a receipt for the taxes paid, i guess in thai so u will have to get this translated. you must file before 31st march i believe. These are new rules so no one really knows the details. i can see this being a total mess. Your saving are taxable also
if u spend longer than 180 days in Thailand u must file a tax return for the work u do in Thailand u will get a tax credit u can offset against any tax in Germany as a result of your work in Thailand. Will be costly to be compliant.