[edit] since photobucket isn't being nice, the pictures are now available here.
https://imgur.com/a/UGJ6G[/edit]
Useful tools

Iron the fabric.
It's best to assume that your fabric isn't exactly square. The easiest way to cut a square/rectangle is to make a template out of card or paper first. Measurements that work for me: 376mm x 314mm. Edit - I also use 15mm seams (quite large, but easier to work with).
Paper template pinned to fabric

Cut out the fabric using the template as guide

The cut out fabric

Sew the inner bag first to save a bit of time later.
Don't start at the very edge, you'll want the reverse a few stitches to lock the thread

Reverse stitch to the edge

...and forwards, removing pins as you go.

Slow down for the corner

Stop and leave the needle down (through the fabric)

Lift the sewing machine 'foot'

Rotate 90%

Foot down

Sew along the bottom edge

Same again for the outer bag - I've marked the seam



Now we need to trim the corners - this will reduce bulk in the seams

Snip 1

Snip 2

Snip 3

... and again for the outer bag (looks like I rotated the fabric by not quite 90 degrees)

Snip 1

Snip 2

Snip 3

Iron the seams open

Turn to right way out, you can use a pointy thing to get nice crisp corners

Iron the seams on the inner bag, we don't need to turn this one out.

Slide the inner bag into the outer bag

Line the seams up.

Pin the two bags together, make sure you don't pin the bag closed.

To get the 'free arm', I need to remove part of the table.

Now we have the free arm, perfect for sewing along the top edge.

Slide the bag over the free arm

Start to sew - slowly. The machine will feed the fabric...

Ideally, the start and finish should meet.

How it looks from the inside

Fold the top edge at the stitching towards the inside of the bag

Iron it as you go to get it to stay in place

Fold it over again, this will completely hide any raw edges

Yet more pins

Sew using the free arm. (for best results. snip that thread before sewing over it)

Final product.





