How do you cleanse before fasting?

What is the best way to cleanse before fasting? What are your food plans/diet before doing a fast?

Is it possible to detox for a few days only and then start a medium length fast?

My body's toxicity levels aren't that high I think. My diet isn't really bad and has never really been. I don't smoke or drink and try to be as active as I can. I want to partake in a fast mostly for spiritual reasons but I'm also interested in the health benefits as well.

I'm also worried about losing a lot of weight so my goal with this fast is to last atleast 10-14 days. Would that be enough for me to see the benefits of fasting or should I extend it to 20 days?

This is also my very first fast so any extra tips are welcomed :)

Thank you!