how to downgrade from high sierra to el capitan

Very hard to provide a simple answer not knowing what computer model and if you have backup and what kind they might be. The simplest way would involve erasing your drive and starting with a blank computer (almost).

About macOS Recovery - https://support.apple.com/HT201314

Use the third recovery option, reinstall the original OS and upgrade to the one you want.

Downgrading and keeping data isn't easy unless you have a clone or Time Machine backup. Do you have one of those?

Revert your Mac to a previous macOS version [with Time Machine] - https://support.apple.com/guide/mac-help/mh15216/mac

https://communities.apple.com/fr/thread/250788825?answerId=251498564722#251498564722

"It is possible to attempt to return to the old system before installation even without Time Machine backup. Mac Os sets up an internal backup system (Snapshots) on special occasions and the system update is one of them.

In short, it is necessary to start in recovery mode (cmd + r during the boot) then to request the restoration Time Machine. There select the system disk (Macintosh HD in general) as Time Machine source and see what is on offer. With a little luck will appear a date / time prior to the update with the correct system version. If so, select it and then start the restore." [Google Translation]

Why not tell us why you want to downgrade, what problems are you

experiencing with High Sierra maybe we can help sort them out.

My original reply was assuming your computer can even downgrade. If it came new with High Sierra it can't run anything lower.