Generate the privatekey ...
cd /etc/ssl/apache2/
openssl genrsa -out exampledomain.com.key 2048
... and change to appropriate rights
chmod 600 exampledomain.com.key
Generate a request by giving the private key
openssl req -new -key exampledomain.com.key -out exampledomain.com.csr
Enter you domain name (exampledomain.com) when you are asked for "Common Name (e.g. server FQDN or YOUR name)". If you want a certificate for all of you domains then entrer "*.exampledomain.com"
Show the contents of your public-key-certificate
cat exampledomain.com.csr
Login at www.cacert.org and create the server certificate (Server certificates -> New) and paste in the contents of the the exampledomain.com.csr file
After the certificate is created paste it from the CAcert-page into a file named exampledomain.com.crt
Apache config
SSLEngine on
SSLCertificateFile /etc/ssl/apache2/exampledomain.com.crt
SSLCertificateKeyFile /etc/ssl/apache2/exampledomain.com.key
Restart apache
/etc/init.d/apache2 restart