This is a very broad question. We’ll gladly give a few suggestions. First, here are 2 helpful links about Eurail (the website is incredible) :

https://www.seat61.com/how-to-use-a-eurail-pass.htm

https://www.seat61.com/interrail-and-eurail-reservations.htm

Starting in chronological order :

You’ll likely land in Paris. Note that the city is hosting the Olympics right during your dates → expect crazy prices, crowds and extra security measures.

I’d head out of the city pretty quickly. Strasbourg could be a good first stop before Switzerland.

After a few days in the Alps (Interlaken and Lucerne are the most touristy areas) head into Italy.

The Bernina railway is stunning and a fantastic way to enter Italy. I’d plan around that. It’s my favourite Swiss railway by far! https://www.seat61.com/trains-and-routes/bernina-express.htm

Italian cities will be very hot and busy in August, I’d spend most of the time in the mountains.

You’ll have to book high-speed trains in advance → peak peak season so they will likely sell out. Have a look at the links I posted above.

To avoid returning to Paris look at open-jaw flights (multi-destinations), you could probably head home from Milan.

Questions welcome. :)