This guide focuses on how to best use the national focuses of your nation to manipulate the POPs to the best composition. To see the mechanics of the pop types and the national focus see national focus and population respectively.
This guide purely considers effectiveness, and does not take role-playing into perspective.
Understanding the mechanics
The ideal composition of POPs
There is not a completely objective ideal composition, but many of the population types have ideal amounts.
Labourers / 
Farmers: There are a maximum amount of POPs that can work in each RGO. Due to the need of other POP types, less than maximum employed people is usually ideal.
Craftsmen: The amount of factories defines how many craftsmen can work. It is ideal that craftsmen make up 80% of the available jobs in factories and not a single one more.
Clerks: Like craftsmen, the amount of factories defines, how many you need. 20% of the amount of work space is ideal.
Clergymen: Clergymen affect literacy increase up to 4% of each state, and research points up to 2% of the total population. Any pop more than 4% pr. state is completely wasted.
Bureaucrats: 1% of the population in each state is ideal. Any pop more than 1% pr. state is completely wasted.
Officers: 0.2% of the total population is ideal to maximise leadership points. Any pop more than 0.2% is completely wasted.
Soldiers: There is no ideal amount other than the more the better.
Aristocrats: The ideal amount of Aristocrats is 2% in each state to increase RGO output, but it is by far the least important pop type, and the ideal amount is below 2%.
Capitalists: Capitalists can invest in factories, and it is thus hard to say how many is ideal. Under planned economy they cannot invest, and the ideal amount is none.
Slaves: Slaves are essentially less effective farmers and the ideal amount is none.
Artisans: Artisans does give some tax income, but on the other hand can prevent industrialization, so it is hard to name an ideal amount, but the ideal amount decreases as the game goes on.
The way, one should think about the composition is that, every POP in a certain role, does not help the nation in any another way. An officer in a nation with more than 0.2% officers, could be either a clerk or a clergyman instead and so forth.
How the national focuses work
National focus is used in a single state to give a percentage modifier to promotion. That means, that e.g. encouraging soldiers without spending on soldiers wages will get you nowhere. National focuses do only support a promotion tendency, it cannot create it itself.
Certain POP types like Officer, Craftsmen and Clerks require a certain literacy rating, so to promote literacy is key to get important promotions.
A nation has 1 national focus pr. 400.000 POPs of accepted culture with a minimum of 1. Furthermore it is additionally limited by culture technology. It is also not possible to use more national focuses, than one have states. The tech in itself, does not have any other content than an additional national focus, but it might be worth taking for the inventions alone. E.g. State and Government has the important Mission to Civilize invention.
Strategy
The first steps
One should always use all of the available NFs, preferably in the largest states possible, in order to have the greatest effect.
The most important factor in a population is its literacy. Its initial level varies greatly, from 80% in 
Scandinavia to 1% in large parts of Asia.
It is important in two ways:
- It increases research points
- It increases the rate POP promotion, which helps the national focuses achieve their intended goal.
Whether one should encourage clergymen to 4% or 2% depends in the level of starting literacy in the province. Above/below 60% starting literacy is a good tell if one should promote until 2% or 4%.
Bureaucracy affects the budget quickly, and will thus relatively easily increase the amount of money that can then be invested in Bureaucracy and education. Therefore, with most nations, one should initially use focuses to encourage either bureaucrats or clergymen, until they are at (or very close to) 1% and 2% (optionally 4%) respectively in all states.
Afterwards
Where one goes from here is depending on the nation's needs.
Industry
If the population is relatively large and relatively well educated (+40%), craftsmen will come automatically when factories are build. If so, encouraging capitalists might be a good idea. If craftsmen and capitalists are not already promoting by themselves, it might be very hard to force them. If one has many employed workers, and a good literacy (+60%) but fewer 20% clerks in the factories, one should consider encouraging clerks.
Military
If one does not have a strong army compared to their nation's size, soldiers might be a good idea. The more soldiers the better, but every POP who is a soldier is not something else instead. If one already has a sufficient military, they might be more useful as craftsmen or farmers.
If one does have lots of soldiers, but lack generals and the percentage of officers is below 0.2%, then encouraging officers might be a good idea. One should, however, not encourage officers if you already have enough leaders.
Other
Artisans and aristocrats help the economy before industrialisation, but are in general never worth it to actively promote. Slaves cannot be promoted, and will all promote to farmers/labourers if slavery is abolished.
Final notes
The industrial, ideological and colonial migration national focus do not work as effectively as one could have hoped. Crisis (and colonization in vanilla) are good alternatives to the promotion of POPs, but the other potential choices are unfortunately lackluster from a strategical point of view.