First of all, before you start trying to send stuff to the interface, make sure to set all the pins of the user port to OUTPUT!

	;Initialize the User Port = Set all pins to Output
	lda #$ff
	sta $dd03

Then set $dd01 to #$00, just to make sure the sync interface is set to do nothing.

	;Make sure the interface does nothing at all
	lda #$00
	sta $dd01

Now you're set up to start sending sync pulses. There are two bits/pins that you use to control the interface. First there is the PLAY bit (bit 2, e.g. the third lowest bit of $dd01), which tells the interface whether the music is playing or whether it is stopped. This should obviously always be set to 1 whenever the music is playing and 0 when it is not. Then there is the SYNC bit (bit 3, e.g. the fourth lowest bit of $dd01), which is what you use to send actual sync pulses. A complete pulse consists of first setting the SYNC bit to 1, and then turning it back to 0 again. So to start playing the music and to send one complete pulse, just do the following:

	;PLAY:  Bit 2 = start and stop
	;SYNC:  Bit 3 = clock pulses
	lda #%00001100
	sta $dd01 ;Set SYNC pulse on

	;Call your C64 music player code here
	jsr call_my_sid_code

	;Now clear the SYNC bit (but not the PLAY bit)
	lda #%00000100
	sta $dd01

Just repeat that code as often as you wanna send a SYNC pulse. If I remember correctly, six pulses correspond to one step in the tracker sequence. When you're done playing and want to stop the music altogether, set the PLAY bit back to 0. In practice you'll just set the whole byte of $dd01 to #$00.

As you have understood by now, it is really easy to use this interface from your own code. Horray for Scannerboy!!!