For the width (if making for a taller / older child), you will, obviously, continue working increase rounds for a lot longer, till you get the width required.
Remember that using dc means that you are increasing the ‘gaps’ and ‘holes’ between stitches.
For a “how to do a fpsc” please check the top of this blog.
When you reach the length needed for your cocoon, fasten off and leave a tail.
Fasten off and leave a long tail to attach.
For the cute lil cap , do visit me at https://shyamanivas.blogspot.in/2017/03/more-mouse-madness-pair-of-minne-mouse.html
Check out some of my earlier patterns for blankets and cocoons.