SmashLadder Launcher
Setting up Replays to watch in SmashLadder Launcher typically only takes one additional step if you've already set it up to open Project Slippi for you, but here is a complete walk-through for the process.
After playing a match while using a Project Slippi build the app will update and display replays like this:
At this point all that you need to do is click the button to set the path...
then select the ISO
After that you will then have a play button available to you!
(There have been reports of a bug where the path doesn't set if you select it this way, restarting the launcher seems to possibly fix the issue)
Click it and your replay should play! That is all!
You can press Tab to fast forward a replay, or the pause button on the dolphin window to pause the replay.
There is currently no way to rewind beyond using save states.
Slippi Launcher
It turns out that @MegaJazz has written a very thorough guide for this process, so it'd be silly to type it up again, if you're using Slippi Launcher to open files you can run through this guide here!