Without a content management system handling defaults for you, DIY SEO on a custom-built site requires a bit more direct technical involvement, but it's still manageable.
Confirm Basic Technical Elements Are Implemented Manually
Titles, descriptions, and structured data all need to be deliberately coded in without a platform generating them automatically.
Build a Sitemap Generation Process
Without automatic sitemap generation, you'll need either a manual process or a simple script to keep it current.
Test Thoroughly After Any Development Changes
Custom code changes carry more risk of accidentally breaking something search-engine-relevant, so testing after updates matters more here than on a managed platform.
Next step: Use the SEO Audit to check your custom site's technical health, since no platform is generating automatic diagnostics for you.