The Nether is a shortcut and a shopping list. Glowstone, quartz, and blaze rods all live there. The door is an obsidian frame with a rectangle of air inside, lit on fire.
The frame
- Make obsidian by pouring water onto still lava, or mine it with a diamond pickaxe.
- Build a rectangle. The inside hole must be at least 2 blocks wide and 3 blocks tall. The usual outer frame is 4 wide and 5 tall.
- Corners can be any solid block. You can save two obsidian that way, for a minimum of ten.
- Crying obsidian looks right and does not count. Use ordinary obsidian.
- Light any inside face with a flint and steel or a fire charge. The purple field means it worked.
Write down both ends
Step through with a block of cobble and a flint and steel in the hotbar. Build a shelter around the Nether-side portal before you wander. If it breaks, you will want to relight it, not hunt for a new one.
On current Java, a spectator can pass through portals too. That is a way to scout, not a way to carry items.