Identify this license plate holder piece
There are two variant of that, which look very similar:

Stud.IO lists them as:


Brick, Modified 2 x 4 - 1 x 4 with Recessed Studs and Thick Side Arches
Brick, Modified 2 x 4 - 1 x 4 with Recessed Studs and Thin Side Arches


These can be seen below, thick in red on the left, and thin in blue on the right in the top picture (not a valid colour):




The difference is subtle but most visible on the side of the bottom, forward facing stud - the picture from the wikihow article appears to be using the Thin variant.