[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.