Head hair and face fur are two different animals, and the experienced barber does not take the tenuous transition lightly. Although the occasional chap revels in marked cranial/facial distinction, most of us prefer that some attempt at blending is made.
One method I've tried:
Make all hairs stand up perpendicular to the skin, starting from the the place where your sideburn transitions to cheek-line (the Elvis hook) and going up to about where the side of your head transitions to dome. Having stood these hairs up, you can (theoretically) look into a mirror head on, see the incongruence in length between the sideburn hairs and side-of-your-head hairs, and use a buzzer to create a gradient from the top of your sideburn to the top of the side of your head.
Suffice it to say that this technique will work when cycling naked is an acceptable alternative to air conditioning.
No, the simplest way to accomplish the blend is this:
Wet your hair and comb all your sideburn and side-of-head hair straight forward, like you're trying to comb it into your eye. You'll notice a leap in length as sideburn hair meets side-of-head hair. Simply create a smooth arc in hair length, from sideburn to bangs.
Now, reverse the process-- comb all hair backward at the same angle, like you're trying to comb it back into your ear. If you can trim around the front of your ear in a seamless arc from sideburn to head hair, while your hair is firmly in this position, you will have come close to blending beard and skull sufficiently.
Alternately, you could just grow both so long that transition becomes a non-issue.