You will need:
A 500 mm long stick. 20x20 mm
A small can
A bit of strong, twinned string
Tiny pieces of hard wood 4x4 mm
Draw a line in the middle of the stick parallell to the edge.
Mark the length of the can.
Cut along the line.
Fasten a nail in the other end. Let it stick up about 15 mm.
Press the stick over the edge of the can.
Fasten it with two small nails.
Make a small hole in the bottom of the can about 20 mm from the edge at the same side as the stick.
Glue the first fret that will keep a distance between the fretboard and the string.
Cut the string a bit longer than the stick.
Tie a washer in the end of the string.
Drag the string and tie it as tight as possible around the nail.
Measure the total length of the string.
Divide the length as the image shows for a blues tuned canjo. Use other intervals if you like.
Glue frets on the fretboard.
Decorate the canjo if you wish.
You can use the washer to tighten the string by turning it.