"Use strips of masking tape. With the cowl off, stick the tape to the fuselage, then lay the tape taut to the glow plug or NV location. Punch a hole in the tape with the tip of the plug or with the needle out, use a pin into the spraybar. Fold the tape back still attached to the fuselage, put the cowl back on then lay the tape onto the surface of the cowl. Location for the hole should be close enough."
I have to add an addendum. Be sure to first draw a reference line on the tape so it can be kept straight. The tape being flexible is prone to not going back in the same place, the line helps. I use a fine tip marking pen. Also very important to keep the tape attached to the fuselage.