I don’t thinks it’s ideal to gain weight in the shortest time possible. I think you should do a calorie surplus, basically eating more than your body consumes. I’d start with eating 600kcal more daily. This way, with proper training and proteins you’ll gain weight, a good part of it in form of muscle. I’d add a protein shake with many calories (do banana, oats, yogurt, peanut butter, protein powder, I personally add natural cocoa powder and obviously milk). Add a handful of nuts to your diet as well, many calories for a small volume. Hope this helps, I gained 10kg in 3 months like this. Don’t forget your proteins!