This is my ingenious and low-tech solution for hanging things like this:
Things that come with two hanging brackets that are not necessarily level with each other, meaning that even if you hammer your nails along a straight line the picture will end up crooked.
Position the item where you want it and hammer in the first nail.
Hang the item on the first nail. Use the level to adjust the item until it's hanging straight. Draw a dot on the wall with a pencil to indicate where the corner of the frame should hit.
Now here's the ingenious part. Remove the picture from the wall. Get a piece of paper. Write "wall" on one side of it.
Position the paper into the corner of the frame. Make sure it's straight. Make sure you're measuring against the second corner, not the first one you've already hammered the nail for. Poke your pencil through the paper onto the bracket, just where you want your nail to go. Now you have a template.
Put the paper against the wall, with its corner right on the dot you drew earlier. Make sure the side labeled "wall" is facing the wall. Use the level to make sure you're holding the paper straight. Now stick your pencil through the little hole you made in the paper. Make a dot on the wall through the paper.
Now you have a mark on the wall right where the nail should go.
Hammer the nail, hang the picture, and everthing should be tidy and straight.
Now our little yarn family has a safe, happy home in the kitchen.