1. Collect 20 obsidian blocks and create a flint and steel using the recipe
2. Go to the place you want your return portal to appear
3. Hit F3 while in the game and collect your X and Z coordinates
4. Plug them into the calculator
5. Use the first 10 obsidian to build a nether portal and step into the nether (cautiously)
6. Explore to the coordinates from the calculator. Again, be careful, the nether is more challenging to navigate than the overworld
7. Build your return portal at the coordinates, step through and with any luck you'll be back at the portal you built initially
Some specific cautions
◆ Dont get caught by very fast moving lava when tunneling in the nether
◆ Mining related falls are really easy if you are moving quickly
◆ One word: GHAST (Bring cobblestone to reinforce any bridgework you do!)
◆ There's an enormous amount of lava. It will burn you up then it will destroy all your gear!
◆ Etc, etc, etc... there's so much to be afraid of in the nether. Be careful!
There are some rare cases in which the gates might not hook up like you expect. If that happens, you may need to do more research to determine what's going wrong. If you stepped out of a new gate in a dark ravine or out in the countryside, recheck your coordinates. Stepping through a gate into unexpected territory is obviously jarring; So, be prepared. It's always a good idea to carry torches, tools, and supplies.
Note: If more than one overworld portal have the same netherside coordinate, the netherside Y elevation comes into effect. Simply place one netherside portal above the other. Fiddling may still be required.