Begin by asking learners what noise a bee makes. As they make a buzzing noise, do the same and put your fingers on your throat, indicating that they should do likewise. This will allow them to feel the vibrations of the vocal chords that occur with voiced consonant sounds. Ask them if they can feel the vibrations.
Then focus on a voiced / unvoiced pair such as /s/ and /z/. Make the sounds with your fingers on your throat, indicating that the learners should do the same. You can help learners with this by getting them to make the bee sound for /z/, and the sound of a snake for /s/. Ask them when they feel the vocal chords vibrate - with /s/ or /z/? The answer should be /z/.
Tell them that this is the main difference between the two sounds, because /z/ is voiced while /s/ is unvoiced. You could then give them a list of words and ask them to categorise the underlined consonant sound into these two categories. With /s/ and /z/, you might choose to include some third person singular verb and plural endings. In this list the sound being focused on is the final sound in each case.
Learners can then use the phonemic chart to decide which of the other consonant sounds are voiced and which are unvoiced:
In a computer lab, learners could do this in pairs. They listen to a sound and repeat it, with their fingers on their throat to check if it is voiced or unvoiced. In a class with an IWB, or a computer and projector, the teacher or a learner could click on sounds while the rest of the class repeat them and categorise them into voiced or unvoiced.
As a follow up, you could do a minimal pairs activity using some voiced / unvoiced pairs, focusing on initial consonant sounds. Display this list or something similar on the board and say a word from each pair. After each word learners have to say voiced or unvoiced, depending on which of the pair they hear. They can then test each other in pairs.
This activity has the advantage of establishing the voiced / unvoiced distinction, and a shared gesture that learners and the teacher can use in class to indicate that a sound is voiced or unvoiced, i.e. the fingers on the throat. It also helps learners to become conscious of the muscle movements involved in voicing a consonant. All of this will be useful in future classes if problems arise in the discrimination or production of voiced / unvoiced consonant pairs.