You cannot make a literal portal to Hell in Minecraft
The closest thing is a Nether portal
Mine or craft obsidian
Build a rectangular frame at least 4 blocks tall and 5 blocks wide
Leave the inside empty
Light the inside of the frame with flint and steel
Enter the purple portal to go to the Nether
In the Nether, avoid lava, hostile mobs, and dangerous terrain
If you want a custom “Hell” area, use commands, mods, or a resource pack