Simple t-tunic for infants

Pleats and embroidery adjust it's size.

Choose a fabric that is of light to medium weight.  Seam allowance is 5/8" unless otherwise indicated.  Cutting and assembly diagrams are below.

This is often called a geometric t-tunic.  Cutting guide:

Enlargement of the underarm section.

Admission here I have to label the sections with a sewing pencil before I cut them apart or I end up with the wrong side up on some of the pieces.
Mark and cut.  yes you only need a piece of fabric that is 18" by 45"

Assembly guide:

Right front

Left Front

Right Back

Left Back

Assemble the quarters as shown in diagrams, in 4 quarters, then, with right sides facing, sew the center seams of first the front and then the back.

This makes a size 2T tunic.  You have two options for making the tunic small enough. 
1. Lay the garment flat and press and pin equal pleats across the front.
2. Or wait until the garment is assembled and install a casing around the neck, thread a ribbon or tie with a section, in the middle of like sized elastic to gather the neck to size.  (the elastic is a safety measure to ensure that the baby will not be choked.

Then , with right sides together sew shoulder seams.   It is much easier to press and sew a rolled hem in the sleeves at this time.

Sew side seams, press and sew a simple rolled hem.  For small babies leave as is, when they begin to crawl and walk either blouse the extra over a belt or baste in a deep hem, letting it out as needed.  If you decide to embroidery the pleats, they can be snipped equally as the child grows.  If you did the casing just sew addition ribbon on to the ends as the child grows and loosen the opening.

I never bothered to make pants, when they are young this fits nicely over a sleeper, or if it is hot weather just a diaper.   As Walker and crawlers I found that a plain pair of snap pants was the nicest looking and the easiest.