Making a homemade guitar can be a fun and rewarding project for a child, but it can also be challenging and require adult supervision. Here are some general instructions for making a homemade guitar:

1. Gather materials: You will need a large cardboard box, rubber bands, a wooden dowel, and markers or paint to decorate the guitar.

2. Cut the cardboard box: Cut the top and bottom flaps off the box, then cut the sides of the box to form a large rectangle.

3. Create the guitar shape: Fold the rectangle in half lengthwise to create the shape of the guitar. Use tape or glue to hold the shape in place.

4. Add the strings: Stretch rubber bands across the width of the guitar, attaching them to the inside of the box on each end. Make sure the rubber bands are tight and in a straight line.

5. Add the neck: Cut a piece of wooden dowel to the appropriate length for the guitar neck and glue it to the top of the guitar body.

6. Decorate the guitar: Use markers or paint to decorate the guitar with your child's favorite colors and designs.

7. Finalize : You can add a sound hole, or tuning pegs or knobs. Your homemade guitar is ready to play!

Please note that the above instructions are for a very basic, non-working and non-tunable instrument, and the instructions can vary depending on how complex you want the instrument to be and the skill level of the builder. Also it's important to note that safety measures like handling sharp tools, hot glue gun and etc should be done with adult supervision.