how did will get in the upside down

Will Byers ended up in the Upside Down after being pulled into the alternate dimension during the Demogorgon encounter, which happened when he was separated from his friends and shaken loose into the parallel world. Over the course of his first week there, he navigated the environment using improvisation and stealth, while the connection between his world and the Upside Down allowed him to communicate briefly through signs like lights in his home. The experience left a lingering link between Will and the Upside Down, contributing to ongoing tension and plot implications in the series.