I have to write many arrangements for this line-up, now the question is: how do I make my own instrument template?
There’s no direct way to ceate your own template in Dorico at the moment, though it’s something we expect to add in a future version. In the meantime, you can do File > Save As to save your current project under a new name, delete the flow(s) from your project, then use that project as a template for starting new ones in future.
yes that is the way I also use now, but in the past I accidentally deleted an entire arrangement and I was not so happy at that time 
but okay thx …