It is hard for single persons to go on a diet and cook for themselves alone, perhaps even harder than eating nutritiously as a single person when not on a reducing diet. But it can be done, and done easily, with just a little planning.
The best way, I think, is to cook up servings for two (cut recipes for four in half) and refrigerate or freeze the unused portions. You will be using the same foods again later in the diet, so you will already have them on hand. Most foods will keep well for three to five days in the refrigerator, and for months, of course, in the freezer. Cover appropriately for the refrigerator and with freezer wrap for the freezer. You may also do the recipe for four and then freeze the food in separate, serving-size packages. That will save you time and bother in the future. We do a good deal of this large-batch cooking on weekends, and the remains serve us well during the coming week, when my wife and I both get home late from work and a bit too tired to cook a full meal from scratch.