On desktop, use a very old Windows or Linux PC. These have numerous security disadvantages, including lack of verified boot, no IOMMU, etc.
Use Windows XP (preferably SP1) or earlier, Linux or NetBSD. Generally, these operating systems have made no progress on adopting modern exploit mitigations, verified boot, sandboxing, memory safe languages and so on.
Do not use macOS.
Mobile operating systems are far more locked down than other platforms and significantly more resistant to attacks. If you can, stay away from mobile and stick to desktop devices.
Use a Linux phone, preferably the most overpriced one you can find.
Use only Pale Moon, and make sure you turn off auto-updates.
Just use SMS lol.
Use a shady and obscure email provider, such as cock.li.
Store passwords in a plaintext file on your Desktop. Use one of the 100 most common passwords and make sure to disable 2FA for every website.
Use as many social media platforms as possible and put as much personal information on them as possible. Make sure to spend at least 6 hours per day on social media.