UPDATE: As of May 6, 2019 it is no longer possible to merge your Fortnite accounts. As confirmed by Epic Games, the Fortnite account merge feature ended on that date, and new requests cannot be submitted for completion. This stands to reason, as Fortnite account merges were only intended to be a short term solution to address the initial cross-platform incompatibility issues between an Epic Games account linked to PS4 and another Epic Games account associated with Xbox One or Switch.
Now, all players can create a single Epic Games account, which can be linked to PC, PS4, Xbox One, Switch, and mobile, allowing access to progress and items across all platforms. As this has been possible for some time, Fortnite account merges should be redundant, which is why the feature is now disabled. If you have old accounts which you didn't merge before the May 6 cutoff, then unfortunately you're too late now and will need to pick one of them to become your main account. It's unlikely this feature will make a return in the future, but if it does then we will update this guide accordingly.
Find some inspiration for your own cool builds with our list of the best Fortnite creative codes.
ORIGINAL STORY: The process does have a few technicalities you should know before starting. Here's the biggest one: the merging tool will only help Fortnite players who are still dealing with the aftereffects of those cross-platform problems. One of the accounts you want to merge must have played on PS4 before September 28, 2018 (when the issues of how to enable cross platform Fortnite matches were finally sorted out), and the other account must have played on Switch or Xbox One before that date. You cannot merge two accounts on the same platform. Additionally, neither of the accounts you're trying to merge can be banned or disabled.
The merger process itself is straightforward: pick the primary account you want to keep using, and the secondary account you want to move all the stuff over from (you'll need access to the email addresses used for each so you can copy in authentication codes). Once you confirm, the PSN / Nintendo Online / Xbox Live logins on your secondary account will be instantly transferred to your primary account.
Don't see all your stuff? Don't panic! You'll have to wait two weeks for the V-Bucks and cosmetic items you had on your secondary account to appear on your primary account. A holding period to make sure you're not trying anything phishy like stealing somebody else's account, I guess.
A little more fine print: not everything will make the jump. The biggest stuff you'll have to leave behind on your secondary account are any Creative Mode Islands you've built and any Save The World mission/level progress (individual items like Llamas and Heroes will transfer). So if you got super into Creative Mode or Save the World on that second "throwaway" account, you may actually want to use that one as your primary instead. Either way, all of your Fortnite Battle Royale stuff will be safe.
Looking for some more battle royale fun? Have you heard of this little game called Apex Legends?