I have no trouble booting to fastboot: I simply press power + vol. up simultaneously and that’s it.
However, I always have trouble booting to custom recovery, in my case, TWRP. For my model (asus X00PD) I should press power + vol. down and then release, however, I don’t know when I have to release or if I have to press power and a bit later the vol. down button, nor do I know if the device only boots to this mode when the usb cable is not plugged in.
advice welcome
You need to read a step by guide and follow the steps.
YouTube a few vids and just follow along. That should answer most of your questions