A plaster cast is a common way of doing this. Pour the plaster mixture into the footprint impression. Once the plaster is dry, remove the cast from the soil. Now that you've got the hang of making footprint casts, try making casts of other objects like coins or shells. A ransom note has been written.
Forensic scientists extract the ink from the envelope so that it can be analysed.
The extracted ink can then be compared with ink from the pens of the suspects. The method used to compare inks is called chromatography. It involves separating the ink in each of the pens. The length isn't really important and will depend on the size of the jar you suspend them in. Draw a small circle 1cm from the bottom of the paper with different black markers or felt tipped pens.