I tried the instructions described in the Blogpost Bootstrap DS215j to install optware for ds215j but was unsatisfied with the long outdated packages it provided. Luckily, there now is optware-ng which is a fork of optware and provides recent packages.
This is how I managed to install optware-ng on my Synology ds215j inspired by the instructions in the Gist Boostrap the Synology DS215j with optware, ipkg, and sudo:
# become root
sudo -i