Being able to jump back and forth in time is a function in many Speculative Fiction stories. Portals exist through which beings and objects can pass. What happens to the traveler can be frightening or placid, depending upon the author. The important aspect is how different the connected worlds are: the greater the difference, the bigger the impact.
Imagine living in our contemporary world, then after passing through a portal you’re back in the Victorian ages? Or perhaps you move forward to find Earth colonized by beings from space? There might be language barriers to overcome as well as differences in available technology. Fabrics and clothing styles would be different as well as boundaries between countries, foods eaten, and types of buildings.
Your task is to write a story in which your protagonist discovers a portal and jumps through. Begin by establishing the normal world in enough detail that the reader understands where the character is coming from, but be careful not to bore the reader with too much information presented all at once.
What does it feel like passing through? Does the character see colors, smell flowers or touch ice-cold barriers? When he arrives on the other side, how does he react? Sensory details will make this world come alive.
Interacting with others before the portal and once in the new world is important. Dialogue allows readers to be immersed in the world without giving them a list of descriptions.
Have fun with this one.