Multistep funnels aren't supported by a custom solution developed by Voluum yet, but there are three ways you can make them work for now. They are all workarounds - and I understand it's not ideal - but since there are many of you who already use multistep landers and get confused with setting up tracking in this specific case, sharing them with you only seems fair.
Our Product Management team is aware of this problem and wants to define it broader so the solution we develop works for a high number of cases. If any of you wishes to be our point of contact when it comes to this feature, PM me or let me know in a post here and I'll be happy to introduce our PM to you. Also, if there is anything else you want to add, I'm here to gather all your valuable feedback - we will go through all of it in the next stage.
I. MULTI-OFFER LANDER METHOD
Let’s assume, that we have 3 landing pages, and the third one redirects to a real affiliate network’s offer.
In this scenario, the traffic flow would look as on the diagram below:

Campaign URL will point to the Landing Page, configured within Voluum as a usual ‘Landing Page’, with ‘Number of offers’ option set to 3:

However, the remaining 2 landing pages have to be configured as offers – so the first offer from the top of the list is our Landing Page 2, and the second offer is our Landing Page 3. The third offer is the real one – configured as you would do it normally.
CTA Button on the first Landing Page has to point to Multi-Click URL no.1 (your-voluum-main-domain.com/click/1). The button on the Landing Page 2 needs to point to /click/2, (to invoke 2nd offer, which is Landing Page 3), and finally, the button on the Landing Page 3 has to point to /click/3, to redirect the user to the final Offer.

If you do not have a static final offer, and need to rotate them, you can mix this method with one of the below, to either redirect the user to a different campaign that will be direct linking to multiple different offers (rotating), or do an internal redirection, to the very same campaign, with a custom parameter and a rule-based path.

II. SECONDARY CAMPAIGN METHOD



III. INTERNAL REDIRECTION METHOD

To get that to work, it is necessary to map a custom variable in the Traffic Source settings:

Then, create a campaign, and set up the default path. Select lander to be your first (A) lander, and the offer to point to the campaign URL itself, with '&step=2' appended to it:

In the next step, create a rule-based path, where traffic with step=2 (custom variable 1 = 2) will be redirected:

This way, you will create a path, where all users who click on a CTA button on Lander A will be redirected:

Please, share your feedback with me and I'll make sure to pass it for consideration while creating the custom solution.