Capturing cookies

Open incognito window (don't use existing session, otherwise you will be kicked from account)
Log in to website you want to convert (e.g. youtube)
Be sure that desired page opens successfully (e.g. you can see your youtube subscriptions/posts/...)
Open devtools (F12). Go to network tab
Refresh page
Open the very first entry
In "request headers" find Cookie:. Copy the entire value to your RSS reader
- Miniflux: Feed settings page, field Set Cookies
- Other RSS readers are not tested yet

That's all! :)