There are 3 requirements for making amiibos you can use on your Switch/Wii U/3DS.

1. 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. EDIT: May 2020 - You can now use Apple iOS phones with the app CattleGrid or the app Placiibo!
2. 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 Gialer.com, they ship very fast from China with very cheap price. the link: Ntag215 1 inch sticker  Only NTAG215 works, 213,216 and other common stickers DO NOT WORK. You can also buy blank Ntag215 card or Ntag215 coin tag in Gialer.com
3. 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?

1. 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.
2. Install the TagMo.apk file on your Android device. You can get the latest version from their github
3. Download these 2 configuration files that work with TagMo: unfixed-info.bin & locked-secret.bin - you can find these by googling their names and at the same places that have amiibo backup .bin files.
4. Place unfixed-info.bin, locked-secret.bin and Amiibo .bin files on your Android device.
5. 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.
6. Touch the LOAD TAG button and select your Amiibo .bin dump file.
7. 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.