Building off of the outline of the Bard class, here is how they cast spells:

Bardic Songcasting

Bards perform their songs by singing, dancing, or playing musical instruments. Songs performed at the Cantata or Concerto level require an instrument.

Songs are performed by selecting a motif, a key, and a duration. The motif determines the base effect, the key determines whether the effect benefits the Bard's allies or harm's his enemies, and the duration determines the magnitude of the effect.

• A Cappella performances require 2 segments* and take effect immediately. After the casting time, Bards may continue to perform and take other actions such as moving, fighting, and being hit. However, they may not cast other spells or invoke other magics without first ending their performance.
• Cantatas require 4 segments* and take effect after the 4 segments*are spent. Bards must use both hands and an instrument to perform a cantata and it requires full concentration that is broken upon being damaged. Bards may only move 5ft per round and take no other actions while performing a cantata.
• Concertos require 8 segments* and take effect after the 8 segments*are spent. A concerto has the same concentration requirements as a cantata.

Bards may perform any motif known any number of times per day so long as the total number of rounds performed does not exceed their Charisma score.

Bard songs have an area of effect of 60ft from the performer unless otherwise noted in the Motiff.

Only 1 Bard song may be in effect at one time. 2nd level Bards may Harmonize two A Cappella effects at once.

Bard songs do not allow savings throws unless noted in the Motif.       

All Motifs are described in the Compleat Bard and also linked to on the right.

*I use the term segments here but not in the strict AD&D sense. Whatever time scale you use, A Cappella performances should take half of a round/action/whatever you use, Cantatas should take a whole round, and Concertos should take 2 rounds.