Also thanks for the ones explaining how to reuse the connectors of exhausted 9V batteries.
Thanks for your time!
Step 1: Elements:
1 2x4 plain brick of LEGO
1 2x4 "normal" brick of LEGO
3 5mm 3V LEDs
1 Connector of an exhausted 9V Battery
You may (will) need a solder an lead.
Of course you need also a working 9V Battery...