DSL are used in runtime and compile time. At runtime they help customising the software through scripts or rules. Systems expose ability to customise the functionality/behaviour through the DSLs. For example rules editor in a firebase - Security Rules language  |  Firebase Security Rules . Quite a lot of google cloud services use Common Expression Language - GitHub - cel-expr/cel-spec: Common Expression Language -- specification and binary representation · GitHub 

The complete guide to (external) Domain Specific Languages - Strumenta covers a lot of things about DSLs.

Elixir has metaprogramming through macros which is compile time.Ecto has DSL for simplifying queries for accessing the data. If your use case is writing a compile time DSL - elixir metaprogramming makes it easy. Runtime DSL in elixir needs a lot of things - like interpreter, etc to be implemented.

Implementing a DSL needs grammar to be formalised as language spec.

What are you trying to achieve with DSL in your application? Is it runtime or compile time ?

More or less, yep. You can’t really learn DSL per se, DSLs are custom languages that you build yourself. For that you need to know Elixir macros.

I recently started the Metaprogramming Elixir book and I enjoy what I saw and tried so far, so that’s one possible resource for you.

DSL is a concept closely associated with Domain Driven Design, the idea is to have a way to easily express things inside a particular domain. For the technical part, you could learn (like dimitarvp said) Metaprogramming Elixir / Elixir Macro. For the design part, you could learn Domain Driven Design (i would recommend Implementing Domain Driven Design for more easier time of reading DDD).