Disclaimer: Always consult a medical or dietary professional before starting any new diet.
Many people start a low carb or ketogenic diet focusing on trying to eat a certain percentage of calories from fat, however, if you are in a calorie deficit the aim is to burn body fat, so you may not need as much dietary fat.
1) Enter your lean body mass, if you don't know, your target weight can be used as an initial approximation. Set the units to Kg or lb as required.
2) Set your energy use, not what you aim to eat. Average values are often stated as 2,500 for Men and 2,000 for women, but this is highly specific to an individual's metabolism and activity level.
3) Set a calorie deficit if you're aim is to lose weight, or set it to 0 calories if you're in maintenance.
4) Set your carbohydrate level. This can vary depending on how strict you want to be, but you may have to experiment. Someone on carnivore may go as low as 0, someone on paleo might go slightly higher but stick to whole food sources, women also sometimes find setting carbs a little higher actually helps them lose weight.
A note on protein:
The lean body mass setting determines the protein intake. The father of keto Steve Phinney at Virta Health recommend about 1.5g-1.75g of protein per Kg of lean body mass. However, protein level can also be very individual specific. If you're trying to gain muscle mass, then more protein may be required. As we age we find it harder to maintain muscle, so again more may be required. Finally, some women also find it better to consume more protein and less fat to achieve the results they desire.