I got this from Norah Vincent's book, "Self-Made Man":
Facial Hair Technique
Use wool crepe hair instead of your own hair. Wool crepe hair comes in long braided ropes, which you can buy from specialty makeup companies. Unwind the braids, comb the strands of hair together, then take the ends between your thumb and index finger and cut them with hairdressing scissors into millimeter-size or smaller pieces. (Or for finer, more realistic facial hair, try using an electric razor.) By cutting them onto a piece of white paper and spreading them evenly across it, you can avoid clumping the hair when you apply the pieces to your face. You can use a large blush brush for your chin and cheeks, and a small eye-shadow brush for your upper lip.
You can use a lanolin and beeswax-based adhesive called stopplepaste, instead of using spirit gum. It's invisible so it won't show through, and it's easier to remove (you need to use acetone to remove spirit gum, which can be harsh for your skin especially if you're doing it on a daily basis). Stopplepaste is easier to apply if you use a hairdryer to heat it for a few seconds before applying it, since at room temperature it's too dense.
Hope that helps! 🙂