Before you start

A job offer from a Thai-registered company willing to sponsor you
Employer meets the ratio rules (broadly ~4 Thai employees and ~2M THB registered capital per foreigner) — confirm with HR
Passport valid at least 6 months with blank pages

Step-by-step

1
Employer files WP3 pre-approval
Your employer applies for a WP3 work-permit pre-approval letter at the Department of Employment (Ministry of Labour). This authorises you to collect the work permit once you arrive.
Via employerWho: Your employer/sponsor~1–2 weeksEmployer-paid
2
Apply for the Non-Immigrant B visa
With the WP3 letter and company documents, apply for the Non-B visa at a Royal Thai Embassy/consulate abroad or via the Thai e-Visa portal (thaievisa.go.th). Enter Thailand on this visa.
OnlineWho: YouA few days to ~2 weeks~2,000 THB single-entry / ~5,000 THB multiple-entry
3
Collect the work permit (within 30 days)
After entering Thailand you must apply for and collect the physical work permit at the Department of Employment, usually within ~30 days of the WP3 letter. It is tied to that specific employer and job.
In personWho: You (employer's HR usually assists)A few daysGovt fee scales with validity (~750–3,000 THB); often employer-paid
4
Landlord files your TM30
Within 24 hours of moving into any address, the property owner/landlord (or you) must file a TM30 address notification with Immigration. You need the TM30 receipt before you can do your 90-day report.
In personWho: Your landlord/property ownerWithin 24 hours of arrival/moveFree (late fine 800–2,000 THB)
5
Convert to a 1-year extension of stay
With the work permit issued, apply at Immigration (Chaeng Wattana in Bangkok) to extend your stay to 1 year based on employment. Get a re-entry permit if you'll travel — leaving without one voids your extension.
In personWho: YouSame day (sometimes 30-day under-consideration stamp first)1,900 THB extension; re-entry permit 1,000 single / 3,800 multiple
6
Report your address every 90 days
Once staying 90+ consecutive days, report your address to Immigration. Window is 15 days before to 7 days after the due date. File online after your first in-person report; leaving and re-entering Thailand resets the clock.
OnlineWho: You~15 min online once set upFree (late fine up to ~2,000 THB)

Documents you’ll need

Passport (valid 6+ months, blank pages)
WP3 pre-approval letter from your employer
Non-Immigrant B visa
Employer documents (company registration, VAT, shareholder list, employment contract)
Passport photos
TM30 receipt (from your landlord) for the address
Work permit book (for the 1-year extension)

Things most newcomers don’t know

The TM30 trips everyone up — and it blocks your 90-day report.

Your landlord (not you) is legally responsible for filing the TM30 within 24 hours of you moving in, but many don't. Without a valid TM30 on file, Immigration won't accept your 90-day report. Chase your landlord for the TM30 receipt on day one.

Source: Immigration Act s.38 / expat-guide consensus

Miss your 90-day report and you pay a fine.

Late reporting draws a fine (commonly 2,000 THB, up to ~5,000). The window is generous — 15 days before to 7 days after — and you can file online once you've done the first one in person, so there's little excuse to miss it.

Source: immigration.go.th / Thailand Starter Kit

Never leave Thailand without a re-entry permit.

Your visa/extension is single-entry by default. Step out of the country without a re-entry permit (1,000 THB single, 3,800 multiple) and your permission to stay is cancelled — you'd have to start the whole visa process over.

Source: official

Remote worker, not employed locally? Look at the DTV or LTR instead.

The DTV (Destination Thailand Visa) is a 5-year multi-entry visa for remote workers — needs ~500,000 THB in savings and a 10,000 THB fee, 180 days per entry. The LTR (Long-Term Resident, 10 years) suits higher earners (Work-from-Thailand needs ~USD 80,000/yr income) and even waives the re-entry permit and stretches reporting to once a year. Neither needs an employer-sponsored work permit.

Source: thaievisa.go.th / BOI LTR

Common mistakes to avoid

Assuming the Non-B visa lets you work — you also need the separate work permit before starting
Letting your landlord skip the TM30, then being unable to file your 90-day report
Leaving the country without a re-entry permit and voiding your extension
Forgetting the work permit is employer-specific — switching jobs means redoing it