I understand most of the technical details, but I don't know where to start working on improvements

That's a really good achievement, and great to hear you're ready to get to work. We have curated a list of existing Models, Frameworks and Charters that might help you structure and plan your work. However, we also developed our own, particularly designed for small presses: 10 Steps to Creating Custom Accessibility Roadmaps

The 10 steps are split into four sections, like this:

A Preparation

1. Accountability

2. Training

3. Identify Objectives

B Analysis and Auditing

4. Baseline Auditing

5. Available and Required Capacity and Budget

C Implementation

6. Documentation

7. Plan Work

8. Public Statements

D Improvements and Benchmarking

9. Improvements

10. Benchmark Auditing

 

And a planning document is available to download here: Strategic Planning Model and Self Assessment Tool '10 Steps to Creating Custom Accessibility Roadmaps'