Have you looked on the Interrail website?

https://www.interrail.eu/en/plan-your-trip/reservations/how-do-i-book-my-reservations

Additionally, there's a thread in this community with additional information on the best ways to book:

In that case, you go to the ticket office, you ask for a reservation and you pay there. That has worked for the past 50 years and will continue to work for the foreseeable future.

And is in fact very much same-same as non-ESpanoles have to do when they wish to use RENFE (as I also did some 2 weeks ago). Plus that your RENFE makes it even more troublesome in their cercanias-systems with closed gates and noone to help you.

The lucky news is that in quite a few countries all this trouble is not even needed; there most trains can be used without any thing to do beforehand (except of course to write them in the travel history). This is f.e. in Germany, Swiss, Austria, Czech, Olanda, Belgium, Denmark, GB  and even a few more.

To newbies using Interrail can seem daunting, especially if you read the many posts and comments on the community.

The reality is quite different and in principle simply reflects the things we take for granted in normal travel.

Your pass is your ticket for travel on the vast majority of trains in Europe. Reservations are available for some trains but they are not included in your pass, although Interrail do offer a service to acquire them for you. Consider reservations like travelling on a low cost airline - nice to choose your own seat but you can save a few pounds by taking pot luck.

Some trains have mandatory reservations but the process is exactly the same for voluntary and compulsory reservations.

When you travel on a train with reservations you will have your ticket (the pass) and your separate reservation.

Just in case you have not worked out how to add a train to your pass (not the reservation) for travel it works through “Your Trip”. Consider this your diary for your planned trains and you can add and remove trains at will. You have only one trip for the total duration of your pass, even if you are planning several separate excursions. 

You can enter your planned trains into your trip from the App in the planner or manually, and easily delete them as well. When you are about to board the train you simply operate the slider to transfer it to your pass and create a virtual ticket. Again simply think of this as similar to going to a ticket office and getting a printed rail ticket before you get on the train. Each train you travel on requires listing in the Pass section of the app, but again you can change your mind and remove that train and add alternatives at your choice.

Also just like any train ticket, if you want to get off at an earlier station for a quick excursion you simply get off and then add another later train to resume your journey.

When I start planning a trip I start with taking a glance at https://www.openrailwaymap.org/, to check where you can go by train.

Then I check the route via planners like DB Navigator, or ÖBB, or SBB or any other international planner.  Start checking appropriate possible routes, along places you want to visit or see. Sometimes it’s good to split up your routes in shorter sections. If you check for trains, e.g. London-Trondheim, you probably get to see weird results.Splitting it up in sections, will give better and more realistic results.

(London-Cologne, Cologne-Kopenhagen, Kopenhagen-Oslo, Oslo-Trondheim for example).

Try to have back-up options and buffer time. They might come in handy with delays - or you might just want to stretch your legs after a couple of hours in the train.

And then, you can start to reserve, with all the tips and tricks the people above this post stated :)

But once you get the hang of it, it’s a very enjoyable way of travelling!

Good luck, and with any further question about specific trains or routes, the community is glad to help!