Fetching:
Generate an API token for the source slack: https://api.slack.com/docs/oauth-test-tokens
Paste the token into fetch.sh
Run fetch.sh
Uploading:
Create directories of images and aliases (or use fetch.sh if moving from another slack)
images/ contains emoji image named with their emoji name
aliases/ contains text files named with their emoji name, containing the name of another emoji to link to
Set the destination slack in upload.sh
Copy your slack cookies for the destination slack
Press F12 in your browser
Click the network tab
Refresh slack
Copy the Cookie value from Request Headers
Paste the cookies into upload.sh
Run upload.sh