I’ve never done this myself but I understand what’s happening.
You don’t cast off anything at all.
You use a different yarn to make the provisional cast on of 3 stitches. Later on you remove this temporary yarn to reveal 3 live stitches which you will pick up. These 3 stitches are the selvedge of your work when you get into the body of the pattern. I’m going to guess that the pattern will say to k3, then a pattern which takes up 8 stitches which you repeat 11 times, then at the end of the row you’ll have another 3 stitches which I will guess are knit. The selvedge is a border either to give a pattern, texture different from the main pattern, and sometimes added to help keep the work flat. If the pattern says to always knit those 3 at the edges it will create a garter edge which will match the bottom edge which is also made in garter. So it will look nice.
So that’s your first 3 stitches on the provisional cat on which later become live.
Garter stitch is knit every row and will be double the number of ridges, 88 x 2 = 176 rows.
Yes it makes a long thin rectangle and when you have your 88 ridges it will become the bottom edge of your shawl. After the 88 ridges you k3 because this is now becoming the edge stitches, I believe it will change direction because you’re going to kind of pull it around 90 degrees (though not harshly) and it become the first 3 stitches of your required 94.
Do not cast them off, these are the selvedge stitches (like those at the provisionalcast on end).
Next, with the 3 stitches still on your right needle you turn the work (rectangle) and work into each ridge of the garter and create a knit stitch. With 88 garter ridges and one stitch created (or picked up) in each this creates 88 stitches.
You now have 3 + 88 = 91 live stitches on your right needle. And will have worked your way across to arrive at the provisional cast on edge.
Next remove the provisional yarn, work into each of those 3 live stitches by knitting into them.
You now have 94 stitches on one needle and the rectangular fabric is now the bottom edge of your work.
So, there is NO casting off, but instead creating a border which should look lovely and I think it will match the edges although I haven’t seen the pattern to know that for sure.
Hope that helps a bit.
Let us know if it works out. It sounds exciting!