First of all, if you need to add a slot in an opportunity you have to be assigned to it so you can have access to the add/edit privilege.
- On the left side of the home page, hover on the opportunities icon and click on the 2nd one "opportunities I manage".
- Choose the opportunity by clicking on the id and make sure that the opportunity is not live.
- Click on "slots" in the side navigation.
- Press on "+ Create".
- Choose your slot details and press "submit".