Installation of Solr

To install Solr version 7.5.0 (assuming no previous Solr installation):

sudo apt-get install openjdk-9-jre-headless
cd /opt
sudo wget http://www-eu.apache.org/dist/lucene/solr/7.5.0/solr-7.5.0.tgz
sudo tar xzf solr-7.5.0.tgz solr-7.5.0/bin/install_solr_service.sh --strip-components=2
sudo bash ./install_solr_service.sh solr-7.5.0.tgz

To create the CKAN core into Solr:

sudo -u solr /opt/solr/bin/solr create -c ckan
sudo rm /var/solr/data/ckan/conf/protwords.txt
sudo rm /var/solr/data/ckan/conf/solrconfig.xml
sudo rm /var/solr/data/ckan/conf/managed-schema
sudo rm /var/solr/data/ckan/conf/stopwords.txt
sudo rm /var/solr/data/ckan/conf/synonyms.txt
sudo mkdir /var/lib/solr
sudo chown solr /var/lib/solr -R
cd ~
sudo ln -s /usr/lib/ckan/default/src/ckanext-dcatdonl/ckanext/dcatdonl/resources/solr/7.4/currency.xml /var/solr/data/ckan/conf
sudo ln -s /usr/lib/ckan/default/src/ckanext-dcatdonl/ckanext/dcatdonl/resources/solr/7.4/elevate.xml /var/solr/data/ckan/conf
sudo ln -s /usr/lib/ckan/default/src/ckanext-dcatdonl/ckanext/dcatdonl/resources/solr/7.4/protwords.txt /var/solr/data/ckan/conf
sudo ln -s /usr/lib/ckan/default/src/ckanext-dcatdonl/ckanext/dcatdonl/resources/solr/7.4/schema.xml /var/solr/data/ckan/conf
sudo ln -s /usr/lib/ckan/default/src/ckanext-dcatdonl/ckanext/dcatdonl/resources/solr/7.4/solrconfig.xml /var/solr/data/ckan/conf
sudo ln -s /usr/lib/ckan/default/src/ckanext-dcatdonl/ckanext/dcatdonl/resources/solr/7.4/spellings.txt /var/solr/data/ckan/conf
sudo ln -s /usr/lib/ckan/default/src/ckanext-dcatdonl/ckanext/dcatdonl/resources/solr/7.4/stopwords.txt /var/solr/data/ckan/conf
sudo ln -s /usr/lib/ckan/default/src/ckanext-dcatdonl/ckanext/dcatdonl/resources/solr/7.4/synonyms.txt /var/solr/data/ckan/conf
sudo ln -s /usr/lib/ckan/default/src/ckanext-dcatdonl/ckanext/dcatdonl/resources/solr/7.4/synonyms_themes.txt /var/solr/data/ckan/conf
sudo ln -s /usr/lib/ckan/default/src/ckanext-dcatdonl/ckanext/dcatdonl/resources/solr/7.4/synonyms_themes_hierarchy.txt /var/solr/data/ckan/conf
sudo service solr restart

If your want to use the ckanext-dcatdonl solr optimizations for earlier CKAN versions it is advised to use the files present in the ckanext/dcatdonl/resources/solr/5.5 directory instead.