Minecraft Speedrunning/Building a Nether Portal
In casual Minecraft, generally players will get a diamond pickaxe, mine 10 obsidian and build a nether portal from that. However since getting diamonds is inconsistent and each obsidian takes around 10 seconds to mine, in a speedrun this is quite slow and a much faster method that only requires a water bucket and some lava. The main idea is to place some water so that lava can be placed to form obisidian in a portal cast. There are two main different areas to build a nether portal in: a lava pool either above or below ground or a magma ravine below which often has lava.
Lava Pool edit
If you're on land, either looking around for a lava pool or digging down for one, this will be the method you will want to use to build a portal and get to the nether.
Here is a simple guide to the most basic nether portal setup, created by former world record holder Cscuile.
Magma Ravine edit
If you've looted a shipwreck/buried treasure and are in an ocean, you may want to enter the nether via an ocean ravine. At the bottom of some ocean ravines are magma blocks which have lava beneath them and if you're crafty you can build a nether portal with it, although this can be awkward with swimming mechanics.
Here is quite a lengthy guide to a few different methods of building a portal in an ocean ravine by T_Wag
Once you've built your nether portal you're headed into the Nether!