How did they recruit? With great difficulty.
That's not meant to be snarky, but it is true. The army would have recruiting stations established in major cities ( often port cities to attract immigrant recruits) and advertise for soldiers in the newspapers. The problem they encountered was that, in the popular opinion, the Army was looked upon as an organization with harsh discipline, low pay, slow promotions, and either filled with the lowest dregs of society, or with people who could not or would not get an job. The lyrics of the song, The Regular Army, O!, contains the line, "Soldier will you work?"
To compound matters, the RA was competing against volunteer units, often comprised of friends, family, and neighbors. While the pay was the same, the local nature of volunteer units (where you could elect your officers) was more attractive than being under the command of a "West Point martinet". Bounties, of course, were an incentive, but a 3 year enlistment vs. an initial 5 year one - later reduced to 3 - was often a deciding factor.
You also have to remember that most people before the war had no knowledge of the RA so almost any negative story was believable. That's why recruiting for the RA was done with great difficulty.
I hope this helps a bit.