Click on your flag at the top left of the screen.
In the stuff that pops up, the first row is “Laws & Government”. The icon on the far left in this row is your conscription law. Click on it and then use political power to change your law. Conscription law from least manpower to most is….
Disarmed Nation, Volunteer Only, Limited Conscription, Extensive Conscription, Service by Requirement, All Adults Serve, Scraping the Barrel.
Each new law on the chain increases the % of your population that can be recruited. It’s not a gradual increase, you just get a huge lot of new recruits dumped into your pool. Bigger is not always better though, as you start to see penalties for training time, construction, and factory output above Limited Conscription.