To put a iPhone 8 or 8 Plus into recovery mode is a little different than in past models.

Have your phone plugged into iTunes and turned off.

The following steps should be be done quickly:

1) Press and release the Volume Up button.

2) Press and release the Volume Down button.

3) Press and hold the Power button on the side of the phone. Keep holding it until to iTunes tell you the phone is in recovery mode.