You save it as a
.ts file by writing the file name extension as
.ts in the save dialog. If this description is not enough for you let me know and I’ll make a screenshot.
After that, you can put the file into the “packages” subfolder of the user TeXmacs folder. To find the user TeXmacs folder the easiest way (works in all operating systems) is to follow the instructions of @jeroen above
then start the procedure of
Save as onto the open file
my-init-buffer.scm. The folder proposed by the save dialog is the
progs subfolder of the user TeXmacs folder, so after that you locate its parent folder in the file system and use its
packages subfolder (where you can create sub-subfolders) to save your style file.
After you do that, TeXmacs should recognize your style file and present it in the list of packages (under
Document -> Style-> Add package); you can then include it when you open a new document.
I think that there is a way to automatically include it in new documents, but I need to figure out how, when I do I shall write it here.
Please let me know if some of the steps are unclear. The aim is that once you have done it, then using TeXmacs with your settings will become automatic under creation of a new file.