If you have an animated gif-file, you can convert it to webm video respecting rules described in the docs (https://core.telegram.org/stickers#video-stickers-and-emoji):
- For stickers, one side must be exactly 512 pixels in size – the other side can be 512 pixels or less.
- For emoji, the video must be exactly 100×100 pixels in size
- Video duration must not exceed 3 seconds.
- Frame rate can be up to 30 FPS.
- Video should be looped for optimal user experience.
- Video size should not exceed 256 KB.
- Video must be in .WEBM format encoded with the VP9 codec.
- Video must have no audio stream.
To convert it, I decided to use this online tool — https://cloudconvert.com/. You can use any other free tool you like. Don’t forget to set the resolution.
After that, you need to visit @Stickers bot (https://t.me/Stickers), send
/newvideo, then send the sticker name, a first video, corresponding emoji,
/skip, a short url name of your sticker pack. That’s it.
To add a new sticker, send
/addsticker and select the pack you want to add a sticker to.