How do I convert cot into day bed

Pillows! Lots and lots of pillows!

Do you have a photo of the cot? A lot depends on the sort you have, so an image would be helpful! Or a link to a similar one.

I cannot see your cot so I don't know what kind of structure you have. Cannot imagine that what I think of as a cot would have any use when building a daybed. Even Pinterest has nothing like you are describing nor does a Google search reveal anything. Here is an instruction on building a daybed: http://www.hgtv.com/design/rooms/bedrooms/how-to-build-a-day-bed

Add some wood-metal to back, headboard, and foot of the bed.

Without knowing what you are working with it's hard to give accurate advise. You could go to yard sales, flee market, antique shops and pick up an old head and foot board of a twin bed and attach the head board to the length of the cot and split the foot board to make arms. The choices are only limited by your imagination.