Basically all you need to understand to gain weight is; Everything is about calories in vs calories out.

Consume more calories than you burn and you will gain weight. Vice versa.

Hence, if you want to gain weight, just eat more than you burn. I'd imagine it would be hard to just suddenly increase the portion of your meals by a large quantity in a short period of time as you would not have developed the eating habit for it yet. Try eating a little bit more on each meal (for people with time constraints).

For example, if every lunch time you eat chap fan (economy rice), try adding 2 additional pieces of chicken to your already existing portion or 2 additional eggs. The idea is to slowly build your appetite so you can handle larger portions of food. Unless of course you're the type that gets full very fast then I'd suggest you go for more frequent meals.

In summary, it's better not to force yourself to eat a lot but actually slowly build the habit of eating a lot so that it will be second nature to you everytime you eat. Note that earlier I said 'eating habit' and not 'forced stuffing your stomach until you feel like puking'. Slowly increase your calorie intake per day and be consistent.

PS: Don't forget to exercise for that "more presentable look" 

This guy has a point