You need Home Assistant running first of all on something. I run mine in a docker container on my Synology NAS.
After that you need to add Octopus integration and get your API keys from Octopus site so it can pull down your tariff data.
For the email to be sent I had to add a OAUTH key for my gmail so it can send emails unattended as me.
All the moving parts not that hard, but those are the 3 requirements. Once all in place I added an automation that triggers daily, reads the sensor for the price, and then uses it in an email I send to me