As with any career in the writing world, the way to gain experience and build a portfolio is to write. A television writer builds something like a portfolio called a “spec script”. A spec script is a sample of writing that showcases a writer's knowledge of the craft and lets others know that he has an understanding of the format of television writing. It could be an original television pilot or a script for an existing show. A good spec will illuminate a writer's skills.
Once their foot is in the door, a television writer is under the gun to write shows that meet strict deadlines. When a writer is creating a spec script he can use as much or as little time as he/she needs. Working in television means a writer has to meet very short deadlines that may change day to day.
Television uses a lot of material. An hour long drama needs a new script every five to seven days. A typical TV deadline takes a week to go from an approved outline to a good first draft. A good writer should be able to accomplish it in four days, which leaves one day to fix mistakes. No one expects perfect work, but they do expect constructive work that can be built on. With such a demanding schedule, a writer is NOT allowed to have writer's block. It's just not an option. A schedule must be stuck to and a writer must be able to power through a script and make it happen.
The most negative thing a writer can do for a network is to deliver a show late. A network has a slot in their schedule for a show. If a show is late then the network will have to find something else to fill that slot with and that means money is lost and people lose their jobs.
Something else writers have to take into account is the internal structure of television. Because commercial breaks are the bread and butter of television, the story needs to be structured with cliffhangers to take place at the end of an act, so that the audience will want to return to the show once the commercials end. A studio's goal is to keep the viewer tuned into the show, and to make them want to return for the next week's episode. It's vital that writers structure episodes in this way to keep the audience interested.