How to get enough calories a day??

The list above is helpful. Think about where you can add chunks of calories that won't fill you up too much. Don't use anything low fat, cook with butter and oil, have some treats...

Something that worked for me was having small meals often plus snacks in between, slowly I found I was able to increase the meal amount and eat more to hunger cues. Calorie dense foods as well with good fats like nuts, nut butters, seeds and avocados. Other calorie dense foods like chocolate and soy ice cream. Smoothies also help cause you can get a lot in them I find. Carbs also good for weight gain protein will make you full fast so while yes include it don't over do it! Soy milk, tofu and other non GM soy products can also help to increase oestrogen because of their natural plant isoflavonoids (if you were to need help with getting periods back)
Also to help your gut foods like yoghurt, miso soup, sauerkraut, tempeh and kombucha have probiotics. Ginger and mint will help with any tummy upsets. When you get to higher weight and are feeling stronger, I would also recommend some resistance exercises as for me I found this increased my appetite (like squats, push ups, abdominal work) or yoga to build muscle and bone mass. But keep in mind that achieving an energy balance is needed so more calories in and less out!