Installation.
There 3 ways to install geshilter in Drupal. For Drupal 10 the recommended way is using composer, as it is more simple to install, but the old way downloading the library and using the module libraries is still available(and is the way to install in Drupal 7).
Full composer:
On Drupal root, run this command, It will download and put the geshifilter module under /modules(or /modules/contrib) and the Geshi library under vendor.
composer require drupal/geshifilter
Go to example.com/admin/modules (replace the example.com with the real name of your site) or you can click on "extend" in the admin toolbar and enable the geshi module.
You can go to configuration now.
Download the module and install the library with composer.
Download the module from the project page https://drupal.org/project/geshifilter and place it in the modules folder in drupal root. You can download it with drush if you want with:
drush dl geshifilter
On Drupal root run this command to instal the Geshi Library:
composer require geshi/geshi
Go to example.com/admin/modules (replace the example.com with the real name of your site) or you can click on "extend" in the admin toolbar and enable the geshi module.
You can go to configuration now.
Download everything(same as drupal 7).
Download the module from the project page https://drupal.org/project/geshifilter and place it in the modules folder in drupal root. You can download it with drush if you want with:
drush dl geshifilter
Download the GeSHi library from https://github.com/GeSHi/geshi-1.0.
Extract it. It will create a folder with some files and a directory with the name src. Copy the src directory to the folder libraries in your Drupal root. Rename the src folder to geshi. Just to make sure: you will have a file drupal root/libraries/geshi/geshi.php not drupal root/libraries/geshi/src/geshi.php.
We need too the Libraries module, download it from https://www.drupal.org/project/libraries and place it under you modules folder. Libraries module is a depency only if you install by hand, using composer we do not need it.
Go to example.com/admin/modules (replace the example.com with the real name of your site) or you can click on "extend" in the admin toolbar and enable the geshi module and the libraries module(need both modules enabled in this case).
You can go to configuration now.