Lying on your back, raise your legs and shift your weight onto your shoulders/neck.

A flatter stomach can make the act significantly easier by reducing physical barriers between the mouth and genitals. Preparation: Targeted Stretching Routine

I understand you're curious about self-fellatio, but it's important to be upfront: this is an advanced physical feat that most people cannot achieve due to human spinal and torso anatomy. For the vast majority, the spine simply doesn't bend that way without risk of injury.