@queenratty     I
've got one of these, and love it.

Re the tunnelling - inside the front drop-down flap where you thread the looper thread  ( blue pathway) there's a little white lever Inbetween the tension disc slot on the face of the machine and the twiddly metal bits ( shown as 5,6,7 on the threading diagram). This lever gives fine adjustment to the looper thread - works wonders if I get a little tunnelling - lever raised = increased tension, lever lowered from centre = tension reduced.
Of course if the tunnelling is bad the tension disc must be gently tweaked.

@Bill
This is similar to the situation when setting-in a sleeve.

A couple of suggestions:

Sew slowly

Try a mock up in scrap fabric, if you've some of similar stiffness so much the better but anything else will let you try your technique.

Make sure that the machine needle is set to the LHS, like you do when inserting a zip - this will help with keeping an accurate SA.

Clip into the seam allowance of the straight edge at several places along the length of the seam.

I would sew this with the curved piece on the top with the straight edge piece nearest the feed dogs, but that's just me and not a rule.

@Andymat
Before you splash the cash, it's a good idea to set up a mock layout of pattern pieces if you can to check fabric required. I do this often, as all of the people that I sew for ( a very chosen few), are at least 6' tall and consequently need length added to most patterns.

Mark out, on the floor or other suitable surface , the width of your intended fabric and layout the pattern pieces as per the diagram in the instructions. You might find that even keeping to the constraints of grain lines and fabric directionality you don't need as much extra fabric as first you feared. Unles, of course, you're stash- building 

The red lines on the clear disc foot, that looks like the USS Enterprise, are supposed to help you maintain a .25" distance from the line that you are following. I've got one. I don't use it. It's a lot easier to practice and do it by eye. The red markings on the foot obscure the line that you're trying your best to follow.

@StitchinTime

Simplicity 1280 is in the post. Enjoy

Is there any chance that this could be made to work without using the plastic bags? I can understand why they are used but I don't use them, ( one of a few ways I have doing my little bit for the planet).

It reminds me of the linen used for making technical drawings eons ago, and was was usually blue.  Hence the term blue-peint.

Washing a couple of times in very hot water would remove the blue plasticy film leaving a very nice fine linen fabric

I've got a roll of this and yes,  washing in extremely hot water does remove the starch. I used some to make baby clothes for DS 30 years ago as he arrived in a very hot summer.  it'd take a lot of the roll to make him anything now 

@Butterfly, No, they don't look like a 'proper' blanket stitch, but they do the same job. If you were doing hand applique you would use blanket stitch  or needle turn the raw edges, the sewing machine gives you alternatives which are just as good.