The first time it's difficult. I was able to get into ketosis by diet alone when I was doing workouts, so that helped a lot to overcome the first 1-3 days (basically, massive headaches on the first day and feeling weird until the 3rd day). Also, breaking the fast was basically a week-long broth and vegetables, then I switched over to mostly salad as food. Obviously, if you don't move, e.g. due to COVID-19, it's difficult to keep yourself in shape; for that I recommend a daily routine of 30-60 minutes on an indoor fitness bike or rowing machine and avoiding sugar/starch.