If you are not familiar with these things, going to a lawyer is safest.
However, that costs money and since they are usually very short agreements you could have a go at doing it yourself. There is a sample here:
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You would need to add a clause dealing with the deposit (presumably paid by the incoming tenant to the outgoing tenant) and the incoming tenant would need to be be aware that he/she would be responsible to the landlord for any damage done by the outgoing tenant. You would also need to check whether the assignment needs to be stamped and, if so, at what rate and agree who would pay that. I'm not sure what else you would need.
Another option would be, if the deal was arranged through a property agent, ask them to prepare the agreement - most of them will have forms they can use.
Disclaimer: this is definitely not my area so there is a good chance that I have no idea what I'm talking about and could have got this very wrong.