Applicants must first create a personal account at my.uspto.gov.
Applying for a registered trademark online is done online.
There are two initial filing options, TEAS Plus and TEAS Standard, where there are more up-front requirements for TEAS Plus but you pay the lower $250 filing fee instead of the TEAS Standard $350 filing fee.
One can apply for either a "use-based" trademark where the applicant provides the appropriate documentation proving that the trademark is currently used in commerce or an "intent-to-use" trademark where the applicant will receive a notice of allowance after which the applicant must begin using the trademark in commerce within a specified period of time.
An applicant can apply for a limited number of extensions to the "intent-to-use" period.