Before you begin, you might want to check out these links:
Painted background: How to paint a sky
How to draw a baby: Adar clown Picture Pie
How strollers work: JugglingFrogs stroller tutorial
Use three 6" paper plates to design your baby's body. Leave one plate whole for the torso, cut one in half for the legs, and cut one into eight equal wedges. Lay out the pieces like this:
Trace this shape onto a large sheet of cardboard. Consider this traced shape your starting point as you draw a more pudgy, baby-like shape.
Cut out the cardboard baby. Squint at it from various angles; if you aren't perfectly satisfied yet, cut and paste bits of cardboard as needed.
Trace onto fabric, adding a good inch all around. (If you are using non-brand-new fabric, position the cardboard strategically so that, for example, stains are either trimmed away or positioned for easy hiding, and preexisting seams come out in acceptable locations, such as at the ankles.) Cut out fabric.
Fold fabric onto cardboard and secure with tape or hot glue. Do this step slowly and carefully.
A bib may be cut from a fancy paper napkin, and is useful for hiding stains (shh!). Other cute options include buttons, pockets, and embroidered initials.