How do I find bastions?

If you aren't opposed to using an external tool, there is a nice site I use for finding both Bastions Remnants and Nether Fortresses. It is called Chunkbase.

All you need to do is select which App you want (in your case, the Bastion app) and supply your world seed. After doing so, the graph will display all the locations of where a Bastion is located for your world - all you need to do is get walking! Note that the tool only works for more recent versions of the game, and also supports Bedrock edition.

The Wiki also has a decent explanation on how these generate:

Bastions remnants won't spawn in Basalt Deltas, but can spawn anywhere else in the Nether.

The regions are 432×432 blocks in Java Edition and 480×480 blocks in Bedrock Edition. Each region has a 4-chunk separation located on the south and east borders of the region in which neither a fortress nor a bastion can generate. This leaves only a 368×368 block section in Java Edition or 416×416 block section in Bedrock Edition where a structure can generate. Two structures will never generate in the same region, however they might overlap if they generate close to the separation border. In Java Edition, the chance of a bastion generating instead of a fortress is 3?5 (60%), while in Bedrock Edition the chance of a bastion generating instead of a fortress is 2?3 (66.6%).

Here is a graphic from the Wiki that represents the above quote:

_{Nether structure generation in Java Edition. The black lines represent each region and the dots represent coordinates. Green is where they can generate and red is where they cannot.}

An extremely easy and useful method one can use, especially in speedrunning, is the f3 method.

Simply press f3, and in the top left corner of your screen you will see an entity count. Make sure your FOV is at 90 first, then start looking around slowly. If the entity count goes to around 40-50, keep looking in that area and turn your FOV all the way to 30. If your entity count is at 20-30, there is probably a bastion in that direction you are looking.

Video for reference.