Are you assessing and recording your caloric intake and your macros currently? Are you assessing and recording your weight daily on a reliable scale? If not, those are the places to start. Buy some scales (body and food), get the MyFitnessPal app, and start collecting data on what you normally eat and how much you weigh over time. If you are losing weight over time, eat more of what you normally eat until you weight stabilizes.
Once your weight has stabilized and you have a baseline, go to iifym.com and develop a weight gain / bulking plan. This will be what you currently eat daily calorie wise + 200-300 calories with prescribed macros. Try to hit your newly prescribed calories and macros each day with a focus on healthy eating (track and record your food intake to monitor this) and keep track of your weight. You should aim for 1-2 lbs per week of weight gain. If you are not achieving that, increase calories by 100-200 per day until you do.
Once your weight has stabilized and you have a baseline, go to iifym.com and develop a weight gain / bulking plan. This will be what you currently eat daily calorie wise + 200-300 calories with prescribed macros. Try to hit your newly prescribed calories and macros each day with a focus on healthy eating (track and record your food intake to monitor this) and keep track of your weight. You should aim for 1-2 lbs per week of weight gain. If you are not achieving that, increase calories by 100-200 per day until you do.