Whole migration mapping is done in yaml files. So there are few things you need to know when doing custom migration configs.
Because of the Configuration Management of Drupal 8, all yml files in the “config/install” directory are only imported when installing the module and then cached. Simplest thing you could do is to reinstall module, there is even in devel module menu item that gives you that option devel/reinstall
but it can be done a bit faster with help of some of these modules below. So look how they can be used and that should help in faster testing of configurations for you migration
also, importing your config again as partial import can help you on the way, something like this
drush config-import --partial --source=modules/custom/migrate_module/config/install/