Inspired by the RL Stine character Slappy.
Made this costume with everything we already owned w the exception of the mask. Wound up finding a George Washington mask for 3.99$ and colored the hair in with a sharpie.

Step 1: Collect Materials Needed

man's suit [suggest suit, not separates to make more cohesive]
mens dress shirt or turtleneck
mask for ventriloquist
towel to be balled up and fill mask
bamboo sticks or similar to support head
backpack to support vent torso and head
overalls for child
stuffing [batting in long socks to use for "puppet's legs"]
child's shoes
child's vest [must be willing to cut back]
swimming "noodle"
long belt or 2

Step 2: Create Puppeteer

stuff backpack with flat cushion [I used patio cushion]. flat gives the ventriloquist a flat back as opposed to rounded.
put men's dress shirt on backpack
insert noodle to fill as arms
feed belts through sleeves of shirt
take balled up towel, insert bamboo stick[s] place mask over and poke through noodle at neck hole

Step 3: Create Dummy and Add to Ventriloquist

shorten / hem pants [can tape up if not willing to cut / sew
use belt to cinch "men's" pants around child's waist
put on dress shoes [these will appear to be men's feet - so a mature style a must!]
place overalls on child w stuffed legs and shoes already attached
folding legs up [have child hold] wrap a belt around child and pinch bottom of stuffed legs in belt. when then flop down they will be short.
put vest on child to hide backpack straps
put suit jacket on vent buttoning just under the legs hiding the bel from 2 steps above
take belts from vent arms joining via belts that were fed through arms connecting under stuffed mini legs. this will give appearance of holding the child

Step 4: Makeup

using the web to find a style of dummy makeup that I preferred. used women's makeup for the following:

cream foundation to give smooth artificial look. a lot of this.
liquid eye liner o create painted eyebrows, lines around eyes & lines on mouth
blue/green shadow just applied to center of eyelids and high enough to brow to see when eyes open creating a "doe" look adding to artificial feel
bright red lipstick sparingly applied because it stains

Step 5: Final Product Was a Hit

this costume cost $4 total. everything else we already owned. lass than 30 minutes to put together though may be slightly longer for people who haven't done many costumes before. was a huge hit with all who saw!