Whilst you are finding your way around the El Capitan upgrade, or possibly still trying to download and install it, I am afraid that Apple has changed the rules, and made it more difficult and costly for you to even test your own content-blocker.
When I was researching this series, you could register with Apple as a developer free, and then register for the Safari Developer Centre, and obtain your own Safari Extension signing certificate, as described in the first article in this series.
Perhaps as a result of the fallout from XcodeGhost, Apple has now tightened up the process, and you will need to pay an annual subscription, and your credentials will be checked. If Apple has any qualms, you may need to provide photo ID in order to complete registration as a developer, and obtain the all-important certificate.
Every copy of Safari 9 can still have the Develop menu enabled, which gives you access to the tool in which you can develop your own content blockers. However you cannot create a usable content blocker without a Safari Extension signing certificate.
To obtain that, the only available route now is to register as a ‘full’ developer (there is no ‘free’ option), for which you have to pay an annual subscription.
If you are already registered with Apple as a developer, it is straightforward to add Safari Extensions to your list of services, thus to obtain the certificate and get up and running.
The good news, though, is that you do not need to upgrade to El Capitan to use or develop content-blockers: Apple has issued Safari 9 as an update to Yosemite too, and the Yosemite version fully supports content-blockers as extensions. But without your certificate, you still cannot pass go, I am afraid.