Write the part for the cello with a bass clef. The bass clef looks like a backwards C with two dots; however, it originated from a stylized backward-curved F. The back vertical line of the F curves backward and the two horizontal lines that form the ledges of the F turned into dots.
Transpose all of the alto recorder notes down one octave. If there is an F4, which is F above middle C in the alto, it will become an F3 in the cello. F3 is directly below middle C on the fourth line from the bottom of the bass clef.
Check to see that all of the notes were transposed correctly. Evaluate the cello part; if most of the notes lie in the middle of the staff or just slightly above the staff, then the transposition is in a good range. If the notes are too high above the staff, consider transposing the part down another octave.