by what means does Milton created a vivid impression of hell

It's pretty grim as you might imagine. There are fires which never end, torture and eternal sadness,

“In the abhorred deep to utter woe;

"Where pain of Unextinguishable fire

Must exercise us without hope of end...."

DEath is also a big theme here, everlasting death in Hell as opposed to everlasting life in Heaven. You also have your assortment of monsters and other dark creepy things,

"Rocks, caves, lakes, fens, bogs, dens, and shades of death,

A universe of death, which God by curse

Created evil, for evil only good,

Where all life dies, death lives, and nature breeds,

Perverse, all monstrous, all prodigious things,

Abominable, inutterable, and worse"