What skills do I need to master to use WordPress effectively?

The answer depends upon what you want to do with WordPress:

1. As a user, your most common activities take place in the admin, and verifying your work on the live site. The skills you need to perform these functions effectively include:
   1. Content authoring, using WordPress editors (easier than most office productivity apps)
   2. Content categorization and tagging
   3. Publishing workflows: Know how to publish (vs setting to draft), trash management, public vs private visibility, slugs and titles, etc
   4. Media preparation and uploading (similar to media prep routines used when creating a presentation or office document)
   5. If your site is using an SEO plugin (I recommend The SEO Framework), you should be able to understand it's recommendations and able to modify content as it advises
   6. If your site has custom post types, you should be trained in their use (well designed post types will be no harder to use than native WordPress editors)
2. As the operator of an e-commerce shop, your most common activities will also take place in the admin, but are also likely to include various third party services (such as payment gateway admins). In this type of role, you are also likely to do verification work on the live site, and the skills you need include:
   1. User skills (as detailed above)
   2. E-Commerce can be complex; know your e-commerce app! Understand all aspects of product creation and modification
   3. Understand shipping, taxes, and be able to manage returns and refunds
   4. If selling products or services that depend upon third party inventories, know how to adjust your offerings in response to changes in those inventories
   5. Access, review, report, and dispute payment gateway transactions
3. As the operator of a specialized app (like an LMS, an events management system, etc), your most common activities are, again, likely to take place in the admin. However, in many cases, you'll find that these specialized apps require additional verification efforts on the live site. The skills you need include:
   1. User skills (as detailed above)
   2. If your specialized app is monetizing via your e-commerce system, you should also have the required e-commerce skills (as detailed above)
   3. Author and maintain all content types made available by your specialized app
   4. Understand how these special content types behave on the live site, so you can properly test them
4. As a project owner, you may spend a fair amount of time outside of WordPress. You'll need to define project parameters and objectives, and to prioritize development and maintenance tasks. The skills you need include:
   1. User skills (as detailed above) are not always required of owners, but an owner that does have these skills will have an easier time with critical steering decisions
   2. Same goes for any applicable e-commerce or specialized app skills
   3. License and membership management
   4. Plugin and service scoping, costing, and selection
   5. Hiring development or infrastructure assistance as required
5. As a project developer, you spend time in every part of WordPress. You build new functionality, maintain existing capabilities, update system components, and more. The skills you need include:
   1. User skills, e-commerce skills, and app skills (as detailed above) relating to all aspects of the system that you are expected to develop, customize, or maintain
   2. PHP (including OO PHP if your work touches any of WordPress' extendable objects, or if you're working in an OO theme or plugin)
   3. WordPress functions and hooks
   4. CSS (and possibly SASS or LESS)
   5. JavaScript (especially jQuery) may be required, though probably less often than other front end technologies and PHP
   6. Browser development tools (Chrome Inspector recommended)
   7. Git, if your project is versioned (as it certainly should be)
6. As the project infrastructure practitioner, you spend time in almost as many places as your developer does (though probably less so in code). You provision service assets, run deployments, and possibly provide DevOps support. The skills you need include:
   1. User skills (as detailed above)
   2. Server management as required in your environment
   3. Project deployment and backup
   4. DNS, CDN, etc

Of course, the above are typical examples, and your specific project may require additional of different skills.