There are 3 requirements for making amiibos you can use on your Switch/Wii U/3DS.
NFC writing capability - generally this means only Android phones with NFC, but apparently it is possible to buy an NFC reader/writer for PC but it is more complicated.
NFC Ntag215 stickers - these are little stickers that have a specific type of nfc chip in them that you can buy for really cheap. I bought mine on Amazon because I didn't want to wait for 1 month to get them in from China. Mine came with 11 ntag215 stickers and got to my house in about 2 days Only NTAG215 works, 213,216 and other common stickers DO NOT WORK.
The TagMo app for Android and .bin files of the amiibo you want to put on your sticker. I'll let you guys google that one, they aren't hard to find.
Okay, so you got your stickers and phone with NFC writing, now what?
On your Android device go to Settings > Security > Unknown Sources (turn on). This makes it so that you can install apps that aren't on the Google Play store.
Install the TagMo.apk file on your Android device. You can get the latest version from their github
Download these 2 configuration files that work with TagMo: https://drive.google.com/file/d/0B7USDVlwa5stY1o5ZF9odXRZeEU/view?usp=sharing & https://drive.google.com/file/d/0B7USDVlwa5stRFZZdmJWQmNsUTQ/view?usp=sharing
Place unfixed-info.bin, locked-secret.bin and Amiibo dump files on your Android device.
Launch TagMo app, touch the 3 dots in the upper right corner > Load key(s) file… and select the unfixed-info.bin and locked-secret.bin files. You might need to install a file explore for your Android device if you can't do that already on your phone, but it should be in the downloads folder if you downloaded it on your phone.
Touch the LOAD TAG button and select your Amiibo .bin dump file.
Touch the WRITE TAG (AUTO) button and press your NTAG215 NFC tag to your Android device.
The stickers aren't re-writeable so I'd advise against trying that in the future so you don't mess the sticker up.
Another ntag215 tag I recommend that works sold on Amazon