28 days is a long time. Maybe start with 3 days and see how you feel. You should prep for a fast by loading your body with nutrients which will be stored for you to use when you are not eating. Eating healthy beforehand makes the fast a LOT easier. If you don't eat healthy beforehand, consider eating small amounts of nutrient dense foods, like spirulina, during your fast to give the body the nutrients it needs for preventing nutrient deficiencies. Fasting is awesome and very healthy and generally pretty safe. I would just listen to your body and if you feel like you are ready for a fast, then just stop eating for a couple of days and drink a LOT of very pure water. Fasting shouldn't be complicated. Our brain (and parasites) make lots of excuses why we shouldn't fast, or why we should stop the fast, but you can push it past these excuses. Food is really not necessary all the time. A couple days is very safe. Maybe that's all your body can handle. Once you have an idea of what it feels like to fast, you can try a longer one. Fasting is really good for you. If it's short, its as simple as restraining yourself from eating, and drinking plenty of water. I'd recommend you get some experience with shorter fasts first, then take a longer fast. It's hard to tell how long you will fast for. Just try it and see how far you go. 28 days without food is a long time. I'm sure it would be very rewarding. Fasting is a serious experience and very rewarding mentally and physically. You might be surprised what you come across during your days without food.