You should start as soon as you can. Typically, you will work on the dissertation for the entire final year. It’s important to use the whole year and not just rush it right at the end as you will need all that time to get a quality piece of work at the end. That said, don’t rush it all right at the start either or you’ll wear yourself and the topic out. You should work on it for a set number of hours per week, just like any other module you are taking. If it’s worth double your usual modules for instance, then assign double the amount of time to it per week.
Keep track of your sources. You’ll need to reference every source you use, or you could be accused of plagiarism. The best way to avoid this is to have a list of every source you’ve used and what you’ve got from it. A good way to do this is in a spreadsheet, including a link to where you found the source, if it’s online, and page numbers so you can easily look back on that specific bit of information. You can then add new sources in very quickly and then use this for when you are writing up your final report, so you don’t forget anything.
You won’t be alone; you will have a supervisor. Your supervisor will be an expert in and around the area you have chosen so don’t be afraid to ask them for help; that’s why they are there. They will be with you for each step and you will have regular meetings with them so they can see your progress. They will also be there to look over your written work and give feedback on it before you submit it. So, use them and their expertise as much as possible.
Whatever the outcome, the dissertation is likely to be one of the things you will be most proud of while at university and remember to enjoy the process.