Advertising and marketing are two different things.
I have run a business of one type or another since I was 18 and am now 39. I've been running a firearm training business since 2013, and rebranded myself in 2015. In that time I have learned alot about what does work and what doesn't work and what works for one industry doesn't necessarily work for firearm training.
The vast majority of my students come through social media, and it costs me nothing but time. I have joined as many local facebook gun groups as I can and give away free advice and answer questions free. Doing this builds trust and gets you seen by hundreds of eyes every day. Eventually people know you know your chit and start asking you questions directly. Congratulations you just EARNED a new student. But it does take time and is an ongoing effort. When I promote a class it generally will fill up quick because people already know me.
But when I need to fill a class quickly, I use google adwords. The problem here is there is no guarantee you'll get students and if you don't do it right you can spend alot of money! ALOT of money. It's not cheap, but everyone uses google.
Remember when I said that advertising and marketing aren't the same? Well they aren't. Advertising is a part of marketing. Once I have a student email address, I invite them to all my social media and add them to my newsletter. I want to make sure I stay in front of them as much as possible. Doing so means that when they think of a firearm instructor I am the first person they think of.