A very nice, and well done video posted in our G+ HamRadio Antenna community by Cory Klumper (NØECK) on how to build center insulators for HF wire antennas.
This video is part of our Youtube PlayList you can browse on the DXZone.com Youtube Channel.
Some additional resources you may be interested on about Antenna Insulators
A center antenna insulator with feedline strain relief
Installing your dipole end insulators, a beginner guide to proper setup wires to insultaors.
Homebrewing special insulators with PVC and copper corona rings