The exact technique to enter / boot into Fastboot on an Android phone varies with the type of device. On HTC (for example), you'll first need to enter Hboot using https://forum.hovatek.com/thread-555.html . For Intel based Dual OS Tablets, you'll get into DNX (Fastboot) mode using https://forum.hovatek.com/thread-18194.html . On very few devices, Volume down + Power button combos will do the trick. You can also get an option to reboot into Bootloader from recovery mode on some Android phones. A generic method (which cuts across most devices) is using ADB.
Steps to boot into Fastboot mode using ADB
- Setup ADB-Fastboot drivers using https://forum.hovatek.com/thread-588.html
- Enable USB Debugging on the phone using https://forum.hovatek.com/thread-10109.html
- Connect the phone to the PC while ON via a USB Cord
- Type the following 2 commands below and press Enter (after each)
- The phone should reboot into Bootloader / Fastboot which'll look something like: