One of the best tips on the art of cushion-making I can pass on is the following: trim all the corners by about .6cm. Taper this sliver down to nothing over 10cm.
(Gotta love my gridded table top!)

This helps reduce the corners from being too long and pointy.
(Gotta love optical illusions!)
To help pin piping evenly around the cushion, I divide the cushion and piping into quarters then match one to the other.
I also make sure that I position my ribbon seam and my cord join on the lower edge of the cushion where they are least visible. To keep bulk to a minimum I make sure that they are about 5cm apart.

Then onto the machine using the same long stitch to sew the cord on. You can see the stitch length in the picture - about 3 stitches per cm.

I then pin the backing on, making sure that the zip is on the lowest edge.
I pin from the backing side, then stitch from the other side using the previously stitched seam as a guide. This seam must have a reduced stitch length (4-5 stitches per cm) and it is important to make sure that this seam is on the inside of the previous seam as we don't want to have any seams visible we 'bag' or turn our cushion inside out.

Make sure you open the zip before sewing as it is difficult to do so afterwards! After this seam the slivers on the corners of the backing fabric can be trimmed off. I dont like to do it earlier as 'things' can 'shift'.

Then onto the domestic to zig-zag

A close-up before bagging out

A light press

A view from the back

And the final product!

I hope this tutorial has been easy to follow.
Please feel free to offer any constructive feedback or to send in photos of your projects to me - I would be delighted to see what you are working on.
If you need specific assistance on any areas that you may be 'stuck' on please free to ask me. Who knows, it could become a future post!