The USPTO must first check whether the minimum filing requirements have been met, before assigning an application serial number and forwarding your application onto an examining attorney. This process could take several months. The examining attorney checks to see if everything is up to scratch regarding applicable rules and fees. A search is performed to see if there are any conflicting trademarks and then the attorney checks the written application and image of the brand.
If, for some reason, the trademark cannot be registered, the examining attorney sends out a letter explaining why. This could just be down to errors made while filling out the form. If you receive this letter, you have six months (from its dispatch date) to reply or else the application will be canceled.
If everything goes smoothly and your application is approved, it’ll be released in USPTO’s weekly publication, the 'Official Gazette'. The USPTO will also send you information confirming the date of publication. Any party who believes that the trademark shouldn’t be registered has thirty days to get in touch. If no-one objects, your application will enter the next stage of the registration process, which can take three to four months. Don’t forget to keep monitoring the status.
If everything is approved, the USPTO will register the trademark and the owner will be sent a certificate of registration. This isn’t the end of it though; the owner must continue filing certain maintenance documents so that the registration stays in place.
The whole registration process can be found in more detail here.