You've got that one friend that talks smack and boasts all the time but, they can't seem to put their money where their mouth is. This oughta get them talking.
To upload and use Discord Stickers you need to have Discord Nitro and a server with at least 1 boost level. To upload and use the What? Are you chicken? Sticker you need to go to the "server settings" page, under the tab called "stickers" you should see an option to upload a sticker. You should click the round plus button then select the image you just downloaded from this website.