The first thing I did was to unscrew the lid from its hinges. I did this because I put my seat cushion directly on the box lid.
You can use a backing board and make a seat cushion that just rests on top of your whole box – as in it isn’t attached – but then you will be lifting it off and putting it back on every time you open the box.
There was no down side for me to doing it directly on the box top as this is a modern piece that I’m not in love with, so glueing and stapling it isn’t a huge risk. If you are working with a more vintage or antique piece, or something with sentimental value, you might want to consider a separate seat top.