does anyone know how to machine tack a quilt instead of quilting. we cannot remember what we did

I think I read somewhere about using the eyelet stitch to do that. There are any number of decorative stitches on my machine that form a single design that I think would work. I just may have to try that!!

Use the zig zag stitch , typically I use the widest width. Take the stitch lenght down to zero or .5. Take several stitches inplace. You can leave long top and bottom threads and using a needle bring the top thread to the bottom and make a square knot with the bobbin thread. FYI ...I almost never do this , taking several stitches is sufficent to hold the tack.
I have also done a tack using a decorative stitch on my machine , stitching only one motif , but that does require knotting the two threads together.
Some use a dot of fray check on the tack, I can go either way on the fray check... as it can migrate a bit and it darkens the place that its used. If you do fray check the tack use a very small opening on the bottle , and just dot both sides of the tack.

I use a thread that either matches or highlights something in the quilt. I set the lengh of the stitch to just barely above 0, and the width as wide as I can (zigzag stitch).

I did a floor quilt for my grandson that had lighthouses on it. I used a very bright gold color and then zigzag-tacked over the light on the house.

Decide where you want your tacks-------each corner of a block, the very center or whatever then take off.

i zig zag wide and short stitch or just straight stitch about a inch forward and back a couple of times

if you look in the siggy blks next to this border you will see some eyelet "circle" stitches. i did one in each triangle corner of each siggy block.

I'm not sure whether I would machine tack a quilt. If you have your machine out, you could just do outline quilting or Stitch in the Ditch if you don't want to tackle FM quilting. I find that the machine quilted quilts that I have done handle repeated washings much better than the tied or tacked quilts I used to make. Tacking or tying allows the quilt to shift more during use or washing and eventually some of the ties or tacks start to pull out or tear the fabric. Some of my older quilts that were tied, I've had to machine quilt to stabalize them.

I have a stitch to sew buttons on. It's basically a zigzag with a zero stitch length. I also have decorative stiches like a satin stitch circle or diamond. I just go slowly and stop at the end of the pattern. I have used both on quilts that I've already stitched in the ditch when I thought I needed a little extra reinforcement.