Activation is explained here:
https://www.interrail.com/en/interrail-passes/interrail-mobile-pass/getting-started
You can do that anytime, but there's no need to do it in advance. In fact many people advise against doing it in advance, because you can only cancel activation before validity has started, which is at midnight. If anything happens after that, e.g. you wake up ill, then it will be too late to cancel and you can loose travel days. You can activate with a start date well in the future, play around for a bit and then cancel the activation again.
Each pass needs its own trip. You can copy a trip though, and add a journey to multiple trips in one go.