Get a microscope with 60 - 100 magnifications
You can find one for 10 or 15 euros on Amazon (cheaper on AliExpress) or at a local grow shop
Analyze the trichomes every day
What kind of high are you looking for?
If you want an active and creative high, you should harvest when about 98 percent of the trichomes are cloudy (whitish) and some trichomes are amber
If you want a more narcotic high, leave the plant longer so that there are more amber trichomes. The more amber trichomes there are, the more narcotic the harvest will be
IMPORTANT: not all buds mature at the same time, the buds at the top and the tips of the branches mature faster, so you can do a progressive harvest: first the top and then leave the plant for another week to harvest other spots
Good luck and ask whatever you need!