Go to Configuration then Order Settings and Printing Templates
You will be able to see the different templates available including Invoices and Picklists
Click on the blue pieces of paper to create an editable copy, it will open up a box and you will have to name it
Then to the right of your newly created copy there will be a blue pen and paper icon click on that to edit it.
Once inside you can edit the template, you can click on the key words or drag and drop them into place there's also the ability to edit the HTML source so you can paste in any designs you may have already have
Once you're done editing just click on the update button and it will save