My number one tip: **drink calories**
I sometimes make a smoothie at breakfast with two bananas, some peanut butter, vegan milk, and a tablespoon of carob. You could throw in any pre-soaked or toasted nuts too. This replaces my morning tea. I often gain about 4kg per month with this.
Generally turning food into drink by way of smoothies enables you to eat more calorie-dense foods easily.