There is no step-by-step guide like the one in Chimera.  Since trajectories in ChimeraX are opened via the generic 'open' command, you should familiarize yourself with how to add a new format to the open command.  This involves writing a 'bundle' that is installed into ChimeraX.  How to create a bundle is described extensively in the ChimeraX Developer Tutorial.  You should read the whole thing, but the "Bundle Example: Read a New File Format" section is particularly relevant to what you want to do.

To make your bundle, I would take the existing code for the md_crds bundle and modify it to support your format.  If your format loads coordinates into an existing structure, it would use the code paths that .xtc and analogous formats use.  If it creates the structure itself, it would use the paths that the .psf format uses.  Feel free to ask questions if you run into problems.

If this trajectory format is private, or is used only in your institution, you can just directly give your bundle's wheel to whoever needs it and they can install it into their ChimeraX with the "toolshed install" command.  If its use is more widespread than that, your could submit the bundle to the ChimeraX Toolshed, where any ChimeraX user can download/install it.  If the format is widely used, you could submit a pull request the the ChimeraX GitHub repository to add support for the format directly to the md_crds bundle.