Writing custom queries

I read these both pages:

1. https://hackage.haskell.org/package/beam-core-0.9.2.0/docs/Database-Beam-Query-CustomSQL.html
2. https://haskell-beam.github.io/beam/user-guide/extensibility/#custom-expressions

And have still no idea how to write a custom query. I followed the first page, which points to the non-existent page https://haskell-beam.github.io/beam/user-guide/extensibility/extensibility. Then I followed the second page and I end up getting errors:

```
• No instance for (IsCustomSqlSyntax
(Database.Beam.Backend.SQL.SQL92.Sql92SelectTableExpressionSyntax
(Database.Beam.Backend.SQL.SQL92.Sql92SelectSelectTableSyntax
(Database.Beam.Backend.SQL.SQL92.Sql92SelectSyntax
(Database.Beam.Backend.SQL.BeamSqlBackendSyntax ctxt)))))
arising from a use of ‘customExpr_’
• In the expression: customExpr_ myFuncImpl
In an equation for ‘instr_’:
instr_
= customExpr_ myFuncImpl
where
myFuncImpl :: (Monoid a, IsString a) => a -> a -> a
myFuncImpl str needle = "instr(" <> str <> ", " <> needle <> ")"
```

The page says:

> Your backend's syntax must implement the IsSqlCustomExpressionSyntax type class.

I don't know what that means. How to implement this? Where's an example?