Guide
Preparing your logs.
Everything I know comes from the combat log WoW writes to your disk. The game still has to write it, and write all of it: I don't guess, I read.
Install the addon
Install SimpleCombatLogger from CurseForge. It records combat by itself in every dungeon and raid, even after logging out: nothing to type in game.
Check advanced logging
In game, open Options › Network and check Advanced Combat Logging. Without it, the log doesn't give players' HP on every hit: no health curves, no replay, and many causes of death stay silent.
A setting to change once and for all.
Where the folder is
In WoW's _retail_ folder, next to the game. With the default install:
C:\Program Files (x86)\World of Warcraft\_retail_\Logs Game installed elsewhere? In Battle.net, the gear next to the Play button opens the game folder in Explorer; Logs is always inside _retail_.
Opening it in Wipe to Win
The easiest way: open Windows Explorer on that folder and drag it onto the analysis page. That's it.
With the “Choose the Logs folder” button, paste the path into the address bar of the window that opens. Chrome then asks for confirmation, talking about files “uploaded from Logs”. The word is misleading: the files are read on your computer, they aren't uploaded.
The next times
Keys already read show up as soon as you open the page, without asking for the folder again. “Rescan” only rereads new or changed logs: count about a second per 100 MB.
A log WoW is still writing may be unreadable at that moment; it will be picked up at the next scan. Warcraft Logs archives (Archive-WoWCombatLog-…) are read separately: pick their folder, warcraftlogsarchive, then tick the ones to analyze.