Gather obsidian blocks and a flint and steel
Build a rectangular Nether portal frame at least 4 blocks tall and 5 blocks wide, with a 2-by-3 empty center
Light the inside of the frame with flint and steel
Enter the activated Nether portal to travel to the Nether
Build an End portal only in a Stronghold using 12 End Portal Frame blocks around a 3-by-3 empty center
Place Eyes of Ender into each End Portal Frame block
Enter the activated End portal to travel to the End
Use a Respawn Anchor with Glowstone to set a Nether respawn point
Build an End Gateway by defeating the Ender Dragon and using the portal that appears after the fight
Use commands in Creative mode to create custom portals if cheats are enabled