Honestly, the tutorials I used are pretty bad (depending on your tech/intuition level) and I just kind of pieced together information from several guides until it worked, but here they are:
https://gbatemp.net/threads/how-to-have-a-homemade-streetpass-relay.352645/#Windows7
I can also give you the basics of how to do it if you have a similar setup to mine (PC with a wired connection and an additional wireless adapter). I may be missing a few steps, since I first set this up for Bravely Default in January, but this is what I had to do last night:
The simplest way is to download Virtual Router Manager and MACycle. The router manager will set up your access point without excessive command prompts, and MACycle will cycle MAC addresses for you in order to get more StreetPasses.
Press your Windows key, type "cmd", right click cmd.exe and select "Run as administrator".
In the command prompt, enter "netsh wlan set hostednetwork=allow" (without the quotes).
Then launch Virtual Router Manager, name the network "NZ@McD1", choose whatever you want for your password, and select your network connection to share.
Next, on your 3DS, go to your connection settings and delete your existing connections so you can add NZ@McD1 as Connection 1.
Here is the point where I often fail the connection test, but I think you're fine as long as the Nintendo Zone icon on the home screen (or wherever you moved it to) starts pulsing.
Finally you run and start MACycle, put your system to sleep, and go about your day (or sleep yourself, in my case).
Enter "netsh wlan set hostednetwork=disallow" in the command prompt when you are done homepassing.
