How can you play RuneScape classic?

You cannot become a Runescape classic member anymore. You have to be a classic member before the conversion to Runescape 2. The Runescape classic is only available to member.

The only ways to play runescape classic is to get a classic account, which is illegal as account sharing/purchase is ban-able and playing on private servers count as third-party, which is also a ban-able offense now, since swiftswitch.