Here is how the tucker goes together. There should be three peices, and one should have two screws in it.
Make sure the screw is all the way up, then tuck the inside edge under the little marker point
and then you sort of press the screw back while pressing the bar down so it will pop into place. BTW - I used my other hand to press the screw back - but couldn't take a picture with both hands on the attachment.
here it is, popped into place
Go easy as you don't want to bend anything permanently. Once in place, you can tighten the screw down - that is how you hold it in place while sewing.
The front guide just slips into place. This one has a little lip on the back side that fits over the back bar - it won't slide fully if this lip in on the wrong side of the back bar.
BTW - while doing this I noticed the copper color scratches - so got out the magnet - and it appears these are all plated brass. The magnet sticks to most of my newer attachments, but to almost none of my old stuff. Does not stick to any of the D8 attachments, and I can tell they are plated brass where the plating is worn. Though my D9 set appears to be steel. Neat to find out more about these items!