1) set FL 340 on FCU (that needs to be done in all cases)
2a: the V/S method) select V/S -500 or -700 and pull the VS knob
2b: the DES method) push the alt-sel knob, commanding a managed 1000 fpm descent
3) read the FMA and have your colleague verify
4) insert the new CRZ FL value into the PROG page.
4 is only a cherry-on-top option if you need to keep things super neat. Completely unnecessary with only 40 NM to TOD. Beyond 100 miles, the machine would do so automatically.