How do I start meal prepping so I can finally lose 4 pant sizes at the age of 65?

You don't have to meal prep to lose pants sizes. You just have to eat manageably fewer calories than your current maintenance calories.

I'm older than you (70 in a couple weeks) but lost weight at 59-60. I'm female, so our sizes are different, but I went from size 18-20 to size 6, which at my height was around 50 pounds. I eat the same range of foods I always ate, just in different portion sizes, proportions on the plate, and frequencies of very calorie-dense choices. I don't routinely meal prep.

Do you meal prep now? If so, look at the foods you usually prep, and consider ways to modify recipes or substitute alternative foods or change proportions of the ingredients, in ways that keep good nutrient density but reduce calories. Logging what you eat now for a typical week would give you a baseline about areas where you're getting a lot of calories, but not commensurate benefits in satiety, nutrition, or happiness with your eating.

Some common general strategies for that kind of thing are to use less oil/mayo, less cheese, leaner meat choices, lower portion sizes of things like rice/pasta/bread, choose lower calorie bread, do fewer fried foods and broil/bake/poach/grill instead, sub plain nonfat Greek yogurt for sour cream, more veggies but smaller portions of meat/starchy sides, etc. There's lots of info on the web about how to "lighten" recipes and meals.

If you don't meal prep now, why do you want to start? (That's not a criticism, just a question. Meal prepping is fine, helps many people. It just would be easier to give advice if I knew why you think it will help in your case.)

If you're currently eating out a lot, it's possible to reduce calories quite a lot by critically considering what you usually order. Even fast food places have options that are lower calorie, and it might not be the items you think. (Fast food or restaurant salads are often surprisingly calorie dense. Sometimes just eating the smaller-sized regular items brings more nutrition and has fewer calories than some of the salads with crispy protein and creamy dressing.) Read menus online, look at calorie counts, plan ahead.

If affordable, frozen meals are also an option . . . though it pays to read labels and consider the calories to protein ratio, and maybe add extra veggies (can be frozen ones, too) to be more filling and get more fiber/micronutrients. A meal service is also an option, if affordable - like one of those things where you choose your meals and they ship to you ready-to-cook ingredients and a recipe. Some even ship frozen meals. In some areas, there are services that will deliver full meals that meet your dietary needs. All of these are on the expensive side, of course.

If you truly want to meal prep, it would help other preppers here if you say what you like to eat. There are so many possibilities!

Some common breakfast preps are overnight oats (prep the dry parts, add liquid the night before); frozen breakfast burritos or mini-frittatas; oat, egg or protein muffins.

Lunches and dinners, you can cook big batches of freezable soups or stews or casseroles, portion, and freeze; make some mason jar salads for some of the early days of the week; make something like a dense bean salad if you like those; prep meat to the ready-to-cook stage and freeze; etc.

One of the things I do is prepare time-consuming-to-cook ingredients ahead, freeze or refrigerate, then use those in quicker meals during the week. For me, some examples are roasting a big pan of mixed veggies (use in soups, salads, stews or sandwiches); making a big pot of dried beans (for soup, bean tacos, use in things like omelets or scrambled eggs); cooking rice or other grains, portioning and freezing (sometimes I freeze on a baking sheet having cut the grain into rough portions, then put the squares in a bag once frozen). But I eat in an unusual style (I'm vegetarian) so those may not fit your needs.

There's lots of info online, or you can search the MFP blog to find quite a few relevant articles, like this:

https://blog.myfitnesspal.com/?s=prep

Best wishes!

PS when I have space in the freezer, my absolute favorite meal “prep” is a giant pan of lasagna. I knead Italian seasonings into a couple of pounds of ground chicken and let it sit in the fridge a couple of days. You’d never guess it wasn’t sausage, and it’s a fraction of the calories of ground beef or Italian sausage.

I use half the pasta called for, but supplement the sauce with carrots, onions, celery, and lentils.

A large pan makes about 28 servings at about 450 calories apiece. It will prep your freezer for months. I’ve learned to buy a crusty roll at my bakery (about 160 calories) and split it between the two of us for garlic bread. Shake some bagged lettuce into a bowl and top with cherries tomatoes and you have a nice feast.

Losing weight doesn’t equate with suffering. It just means paying attention, learning substitutions, focusing on changing your taste buds, and occasionally saying “OH! I can make and use homemade labneh” instead of Philly Cream Cheese?!” (17 calories versus 60, 100 or more? I haven’t used Philly in years!) and still enjoy good taste.