Tune each cellists strings to the same pitch. Use a tuner to ensure that all of the strings are playing in tune. Tune the lowest string first, then the second string and then play both strings together. Continue to the highest string until every string is tuned to the same pitch.
Play long tones at the beginning of the sectional rehearsal. Ensure that cellists all play with the same bow length and same volume. Have the section leader sit in front of the other cellists so others can mimic the movements.
Analyze the music with the section. Tell them where the accents should occur and have them write in slurs over the notes you wish to have bowed with a single bow stroke. This will increase the clarity of the sound and ensure the cello section plays the same up-bows (pushing the bow over the strings) and down-bows (pulling the bow across the strings). Indicate an up-bow by a "V" shape above the note and indicate a down-bow with a sideways bracket.
Practice each melody with the cello section until they're listening to each other and playing each note of the melody in a similar fashion. If necessary, have the lead player demonstrate how the melody should be played. In professional groups, this shouldn't be necessary as the other cellists will know how to listen to the lead.