IF this is your very first fast, doing a 40-day Water Fast may not the best weight-loss option, and may be dangerous.
Best to do a 90-day juice fast with chlorella. There are plenty of YouTube videos of people losing 80-100 pounds this way. Plus, this is easier and certainly safer.
If you really want to do water fasting, then start with just 8 days, then refeed on fruits and vegetables only, then a longer one, and so on.