If you are planning a live music event you will need to make sure you have a contract in place with the performers. This is especially important if you have a financial stake in the event and are inviting paying customers. You need to make sure you have a written guarantee that the cover band will attend the engagement and what is expected from both parties. There may be occasions with local function bands where a contract is not needed. Sometimes an email confirmation may be enough if you know of the act.
If you do go down the contract route the live band may issue you a standard contract with their terms. These are available from the Musicians Union and are very simple straightforward contracts. It will usually include as a minimum the date and time of the engagement, the location, fee and duration of the performance. More in depth contracts may include who is responsible to providing equipment such as staging and PA systems. Some cover bands will be paid a set fee for the gig and other larger acts may ask for a percentage of ticket sales as well. This is something that should be negotiated early on.