Duvets are very similar to sheets but you begin with two layers of fabric instead of one and they aren't ever fitted.
One problem you will have is that the fabric is often distorted from its original shape. Accept the fact if it is distorted and don't try to get it perfectly square, you'll just waste lots of time and in the end you still won't have a square duvet.
Start at the top end of the duvet, the part closest to your face when on the bed. Halve it making sure the right side is on the outside (the right side being the one you would show uppermost on the bed).

Make sure the material is flat, stretching the layers by putting you hand and arm in the center and pulling against your other hand. DO NOT iron until all layers are flat.

The easiest way to flatten the fabric is to use the tips of your fingers to roll any creases over the back edge of the board. Here you can see a roll being pushed over the edge. Do it again and again until all the material is flat. Push sideways as well and over time you'll get used to getting layers flat.

Repeat this section by section until you reach the bottom of the duvet. Iron with the point into the sections between any buttons or poppers.

Going over poppers and buttons with the iron can melt or break them so it's not a good idea.

Fold the top end up to meet the bottom end so you halve the duvet.
Then grab both ends and shake to see where the middle is. Take the center laying it flat on the board and start smoothing down the material again. Pull it towards you section by section making sure all the layers are flat before you iron. When you come to the end you can either do the same again, folding down to eighths or just flip the whole side over and complete the side you haven't done. If you fold the duvet down beyond eighths creases will appear where it's folded.

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