Using a needle with a plastic cannula over the top of it, you slide the needle into the vein and then slide the cannula off of the needle into the lumen of the vein. Upon complete insertion you will then occlude the vein, and withdraw the needle at which time you will connect your extension tubing to the plastic cannula. That explanation is quite simple, but the procedure and all things surrounding it are a little tougher to learn than just reading it out loud.