Let's make a paint cup from an ordinary index card. I use this method because it's the old timer's way of doing things, and that's what makes me tick. And index cards store neatly and don't take up a lot of room in your paint box.
Take an ordinary index card,...
Take an ordinary index card, and fold it into thirds lengthwise and then fold it into thirds crossways. Unfold the entire card and it should look like this.
Bend one end of the index...
Bend one end of the index card up like this.
Here's the tricky part. Bend...
Here's the tricky part. Bend up one side of the card, creasing the corner square on the diagonal. Flip one end up between my fingers and fold one outside corner to the middle corner.
Fold in the triangular flap....
Fold in the triangular flap. It should now look something like this.
Do the same thing in the other...
Do the same thing in the other corner of the card.
Bend the resulting rectangular...
Bend the resulting rectangular flap over the edge.
Repeat this at the other end...
Repeat this at the other end of the card, and here's what your paint cup should look like. Wasn't that easy? I can do this in 10 seconds.