The instructions are talking about the seam allowances. Sometimes the end of the elastic you are feeding through gets stuck in one of the seam allowances so you have to yank back a bit and refeed the safety pin or bodkin through the area. This is just one of their little hints to anchor those ends down. The way I look at it is by the time you get through doing all that you could have the elastic fed through. It does pay to pin the loose tail to something so you don't slurp that end through the casing and have to fish it out.

What they mean is that if you use fusible web to glue the seam allowanced down in the casing area, it's easier to run elastic through it.

There's a product called Steam A Seam that works well for anchoring the edges of the pressed-open seam. It comes in several widths (1/4" and 1/2" that I know of). Iron it to the edge of the pressed-opened seam that will be inside your casing, then remove the paper backing and iron again to anchor the edge to the garment. Then, when you thread the elastic through, it'll prevent the safety pin (or whatever you use to thread your elastic) from getting hung up between the two layers. Or you can machine or hand baste along the edges of the seam allowance to do the same thing.

Mary