What can I do to fix my saggy butt? Lost a lot of weight. (Photos)

 First, congratulations on the weight loss. However unfortunately once the skin is over-stretched it is unable to recoil back, even with healthy nutrition and exercise. Usually what I recommend in these cases is to "reinflate" the buttock first by placing a buttock implant restoration of volume and fullness. Then  6+ months later remove whatever residual skin remains with a "lower buttock tuck". Glad to be of help.

Hello dear!
Thanks for the question and provided information as well.
Based on your pictures, you are a good candidate for liposuction with BBL and buttocks tuck, to remove the extra skin, and I think you'll get very good results. 
Liposuction allows your plastic surgeon to transform your entire shape. You can improve your abdomen, hips, back and flanks, and of course your buttock, to give you a beautiful heart shape.
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)

It is very difficult to determine the exact size and shape implant you will require to best match your ideal butt image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your buttock will determine, in many cases, the maximal volume per implant profile that you can accommodate. Liposuction of your lower back and fat transfer is also helpful as an adjunct to implants. Using “want to be” photos however are useful if simply provided to the surgeon.