Leave this until you've done everything, lvl 50+ and have collected a ton of junk.
Amount of people at settlements is 10 plus your charisma. You can boost your charisma using clothes items.
Hat - I used minutemen general hat +1
Glasses - I used black rimmed (I think fashionable is the same) +1
Suit - Reginald's outfit (I think there's a dress that might be the same level) +3 (there are plenty of +2 bonus suits)
Using the above and my base of charisma 9 I could get 24 people at settlements.
After trying to use Sanctuary and then The Castle as settlements to get to 100% I gave up and started from scratch using Starlight Drive-in.
At Starlight you can get rid of the radiation in the water by trashing the barrels.
Go to another settlement (the one you've been using to collect junk), select a person to start a trade route to Starlight...send a few random people at other settlements to Starlight too if you wish to speed up the process of population growth.
At Starlight setup a recruitment beacon, create 25 beds undercover (make more covered space if you need to). You can just create sleeping bags as they have a slightly smaller footprint (better for tight spaces) and the bed type doesn't seem to make any difference from what I can tell.
Place 24 mutfruit plants for food and set 4 people to work them.
For water I just made a large purifier put it in the water pool with an appropriate generator nearby.
Create machinegun turrets and build up defence to 100 or more (I also had one rpg turret but that requires power).
For all the other people (20) build 3rd level shops, mostly medical type but I did have one weapons shop...you'll need a couple of upgraded perks for this.
The reason I had a weapons trader was to make use of the trader glitch, so I could access extra cash to build most of the shops and wait in the settlement while happiness went up (this last thing is important, if you leave happiness will go down).
After you've taken care of beds, food, water, defence, all the shops and have power (I only had 30) you then max out the settlement size bar by going around putting down rugs, cover every wall space with paintings, have a few chairs around, a few tvs (need power) and I had around 4 radios (don't need power) tuned to Diamond city radio.
I also had all the crafting station types but I'm not sure if that's relevant. I just wanted those there for myself. Plus I had a computer console to access holotapes.
Look at the settlement happiness. If it has an up arrow then wait.
When it stops going up, sleep for 24 hours.
When it gets to 98 and 99 it seems to take forever to move but it will.
Extra notes:
- While you're doing all of this do not leave the settlement for any reason, even just briefly to engage attackers or something like that.
- When I did this I didn't have any companions at all. I'm not sure if that's relevant (like for population) but thought I should mention it just in case.
- Make sure you have a couple of benches and chairs around, you will need to use one yourself to wait. Unlike earlier games that had a wait button, in F4 you sit down in a chair then you get a wait option prompt.
- Make sure you have enough sleeping bags (or beds if you want to waste a tiny bit extra space and resources) for every person plus one extra for yourself. This way when you sleep you won't interfere with any other person. I have read that sometimes players have seen the population stop what they've been doing and just stand around not doing their assigned tasks...this might be due to not having the extra bed for the player, or maybe it's irrelevant and I was just lucky.
Fallout 4 fest - major spoilers hidden please!
I think I've done 3 already on this save I'm using atm but I can't be sure as I used multiple saves for different main quest lines.
These bastards are a lot tougher now, plus they create a fair sized explosion when they die.
I'm lvl 53 so 
