My experience was that implementing/modifying an existing (basic) fpga ethernet stack is pretty doable, since you quickly get an indication that something works and there are very distinct layers.

The colorlight 5a-75b together with Litex is a great way to get started for cheap.

If you want something even more basic, https://github.com/kingyoPiyo/Pico-10BASE-T is very simple.

Layout for most consumer standards is pretty lax, almost anything will probably work.