The end of the Polyleader terminates with a short section of monofilament. From this point you will want to add some more tippet material before tying on your fly. You can either create a loop in the monofilament of the Polyleader and then loop on tippet or you can use a blood knot or albright knot to join the two.
In terms of the length of tippet to add, keep in mind that you want your sinking Polyleader to pull the fly down in the water column. If you use too long a section of tippet then the Polyleader goes down and the fly stays up. Not good.
In general, 3 - 4 feet of tippet is ideal. It is long enough to separate the fly from the Polyleader but short enough to get the fly down.