Minecraft wiki entry for Nether_survival, associated text: The minimum size for the frame is 5 high by 4 wide, which requires 10 blocks of obsidian and produce the "classic" nether portal 3 high by 2 wide within it.
The corners can be any block (including more obsidian) or none at all.
The frame can be larger, as long as it is rectangular and there are no other blocks inside the rectangle.
A ruined portal can provide a partial frame for the player to complete, but any crying obsidian must be removed from the frame, as that is not a valid frame block.
The "traditional" way to create the frame is simply to get 10 or more blocks of obsidian (mined or from loot chests) and place them into the frame shape.
Alternatively, a portal frame can be partly or completely "cast" in place, as described below. This is useful if the player does not have a diamond pickaxe.
Once the frame is built, a fire block anywhere within the frame activates the portal.
The traditional way to light the portal is with a flint and steel.
A fire charge also works. Fire charges require a Nether material to craft (blaze powder), but even in the Overworld they can be found in a ruined portal chest.
Lacking either of these, any other source of fire works; if wooden blocks are placed facing the portal (but not in the frame) lava can be used to set those afire, providing the needed fire block.