This page of the Disco Elysium guide contains a detailed walkthrough of the Split a 'kilo' with Cuno side quest.
In the Split a 'kilo' task with Cuno quest in Disco Elysium , you have to do a task for Cuno and sneak into his apartment to steal his father's speed.
After earning his trust, you'll be able to talk to him about drugs. Cuno will reveal that you can find some speed in his apartment that belongs to his father and he will offer you a deal: if you can get speed for him, you will get half of it.
Make your way to room 12 of the Capeside apartment building and open the door using chaincutters.
Go to the desk, open the drawer and take the speed from the table.
You can still have a look in the next room where Cuno's father is sleeping (optional). The man will be unconscious and you will not be able to get any information from him.
Go back to Cuno and decide what happens to the drugs- there will be many different dialogue options to choose from. If you stay fair and stick to the original version of the contract (Alright. Fair is fair), Cuno will measure two doses of speed and will hand you a vial.