I tried OctoSlack and Slack - ran into some problems getting OctoSlack working, honestly don't remember what they were since I was able to get the Slack plugin working without any troubles, so I uninstalled octoslack. If I remember, it looks like OctoSlack has more features so I can understand wanting to go that route instead, but it's an option.
(slack.com) Either create a new workspace or you need to have admin rights on an existing one. Note that free Slack limits you to a total of 5 channels.
(slack.com) You probably want to have a channel which is just for your octoprint notifications - if you have multiple octoprint instances you can use multiple channels. (see above note about free vs. paid Slack - of course you can have multiple octoprint instances reporting to the same slack channel)
(slack.com) Add an app (this is where you need admin rights). You want the bot app. You have to associate the bot with a channel (as mentioned in step 3). Copy the bot token.
(Octoprint) Install the Octoslack plugin if you haven't already done so.
(Octoprint) Go to settings / Octoslack, select the first Slack option (Slack app bot token), and put in the bot token you copied from your Slack account (Slack Bot Access Token). You'll also need to put in the name of the Slack channel from step 3 (if it shows up as #printjobs in Slack, enter printjobs here without the hash). You almost certainly want to check the restricted checkboxes for commands (if you want to leave those enabled) and put in the slack usernames of those who will be able to issue commands (such as cancelling your print job).
Everything else is straightforward (which events get reported).