Can I do this if I've already stayed in Thailand for 6 months?

Highly unlikely. Thai immigration tracks your entries. If you have spent more than 180 days in a calendar year, you will almost certainly be denied.

Do I need to stay in Cambodia overnight?

No, but it is often safer to do so to show a legitimate reason for crossing. If you return within minutes, you invite extra scrutiny from border guards.

Are there scams at the border?

Yes. Avoid anyone offering to 'fast-track' your visa or asking for 'processing fees' higher than the official government cost.

How to execute a land border visa run from Bangkok to Cambodia

Take a bus or private taxi from Bangkok to the Poipet border crossing, walk across the border, get your Thai exit and entry stamps, and immediately return. This process resets your Thai visa-exempt entry or tourist visa, but be aware that Thai immigration limits the number of back-to-back land entries allowed per year.

Book your transport. Book a bus from Mo Chit Bus Terminal to Aranyaprathet or a direct van service from Bangkok. Avoid 'all-inclusive' visa run services that offer to handle your paperwork for a massive fee; they are often scams.
Clear Thai Exit. Walk to the Thai immigration building at the border. Queue up, present your passport and TM6 departure card (if you still have one), and get your exit stamp.
Cross the border. Walk past the casinos towards the Cambodian immigration office. Obtain a Cambodian visa on arrival if you don't have an e-visa. You will likely pay roughly $30 USD in cash.
Re-enter Thailand. Walk back to the Thai immigration entry desks. Present your passport and complete a new arrival form. Ensure you have proof of onward travel (a flight out of Thailand) as they may ask for it.

Can I do this if I've already stayed in Thailand for 6 months?: Highly unlikely. Thai immigration tracks your entries. If you have spent more than 180 days in a calendar year, you will almost certainly be denied.
Do I need to stay in Cambodia overnight?: No, but it is often safer to do so to show a legitimate reason for crossing. If you return within minutes, you invite extra scrutiny from border guards.
Are there scams at the border?: Yes. Avoid anyone offering to 'fast-track' your visa or asking for 'processing fees' higher than the official government cost.