Hummer, to make it simple?
You simply need to know the following:
1.Developed countries: Have a narrow base and wide middle and ending.So, basically the pop. of the youth of between 0-4 are very few and from 5-9 are a bit more ( altho still few ) and so on. TBH, its random but the basic thing is that the range of children are low and grown ups are high cuz they have low birth rates and low death rates.
2.Developing countries: Have a wide base and a narrow middle and ending. So, the pop. of the 0-4 children are very high since the birth rate is real high and the death rate is high too so for old people simply draw a narrow population structure for the older people (eg: 60-65)
I didnt understand the latest post but heres what I understood:
Migration consists of basically people coming into the country (Immigration- which could be remembered from I for In) and people going out of the country (Emigration- opppsite of Immigration)
Net migration: Immigration - Emigration
Hope this helped