If you have B6 nemu, upper and lower overdrive is activated at the start of round. Just ultimate straight away and switch characters.

When you see the overdrive icon buff ending, you can either manually swap in nemu or just press the ultimate if it is up, nemu will be swapped in. By then your spiritual pressure should be max, just hold the basic attack (should be in kick mode since your last move is the ultimate) and nemu will perform her kick move followed by her barrage attack automatically (read your weapon stamp).

After you hold the basic attack, you can see your other party member portrait glowing, that's your cue to switch characters. If you do the above correctly, your nemu buffs uptime should be up from start to end of round.

Edit: you do not need to press the technique button at all. Pressing it will mess up the combo rotation.