You will need:
A 500 mm long stick. 20x20 mm
A small can
A bit of strong, twinned string
Some nails
Tiny pieces of hard wood 4x4 mm
Glue

Step 1:

Draw a line in the middle of the stick parallell to the edge.
Mark the length of the can.

Step 2:

Cut along the line.
Fasten a nail in the other end. Let it stick up about 15 mm.

Step 3:

Press the stick over the edge of the can.
Fasten it with two small nails.

Step 4:

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.

Step 5:

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.

Step 6:

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.