The most effective way to minimise accidents is by identifying potential risks and working out a way to try to mitigate them. The exact risks applicable to your event will vary depending on a number of factors but you should consider what to include carefully.
Your assessment should list the potential risks you could encounter from the point your event begins to the point where it's completely over. You will then be asked to evaluate the likelihood of something happening against the severity of the outcome if it did.
For example, at an event held near to open water, the likelihood of people falling in may be fairly low - but the severity of the outcome if they did could be quite high.
You will then be asked for control measures you will put into place to mitigate against the risk happening - this may not remove the risk, but it shows how you will attempt to reduce it.
Finally, you'll need to re-evaluate likelihood versus severity, once those mitigations are in place.