First, screw up some knitting to the point that ripping it out would make you cry. Misreading the pattern is recommended. Putting some intarsia between you and the error is recommended but does help increase aggravation. (The following is a dramatization and is not a real project).
Second, pick the stitch you are going to ladder down. I recommend the one furthest away from the edge... if that is a factor.
Next, drop the stitch and slide all of the rest of the stitches onto a stitch holder. Now tug on both sides of the dropped stitch until it ladders to the point you want to catch it. Stick a needle in the stitch you want to catch, just in case you get a bit too excited with the laddering.
Replace the needle with a crochet hook, hooky bit facing you.
Since you want this to be garter stitch, you have to alternate the direction from which the loops pull. The easiest way to do this, in my opinion, is to connect loops stockinettely on alternating sides. The first stitch will be stockinette on the wrong side, so stick the blunt bit of the crochet hook under the first ladder, flip your work so that the wrong side is now facing, and push/pull on the hook until the loop pops out in front of the ladder and you've caught the active loop with the hooky bit.
Now point the groovy part of the hooky bit downwards (toward the bottom of your work), come over the ladder from front to back, twist the ladder around the hook, and yank it on through the loop actively on the shaft.
The hooky bit should be facing you again. Now pull nearly the whole length of the shaft without dropping the stitch off the blunt bit.
Find the next ladder, aim the blunt bit of the shaft under it, and push it back towards the other side, flip, and do the stockinette loop shuffle again, this time on the right side.
Repeat until you get back to the top. (That's only 50+ rows, right?) Then repeat for each stitch you want to be garter stitch.
So I did this with three stitches and goodness knows how many rows per stitch (probably around 30). My project went from this:
Look, it even got less blurry! It does work, and it looks a lot nicer than I thought it would. You actually can't tell the difference between the rows I knitted this way and the ones I shuffled. So while I'm quite pleased with the results, I will make sure to read the directions more carefully next time.