Myandrin

I have realy no clue how to design Turrets by my own. Have watched some YouTube content but my problem is maybe realy simple. I cant start the design. Im not sure what im doing wrong. I klick middle mouse on a set Turret to mark it. then in Turret window upper left (the turret symbol) open design modus. Game says, choose a turret base block for creat you design. I marked the block conected to the turret, same. Tested some other stuff, didnt get to a result. Would be nice if anyone can answer this maybe simple question. Thx, Myandrin