To import your current Slack data into Rocket.Chat:
Go to https://[your_app].slack.com/services/export
Click on Start Export
Wait until your download is ready and download it (it will be a .zip file)
Go to https://[your_rocketchat]/admin/import/new/slack
Select "Slack" as Import Type
Choose your downloaded .zip file and wait until Rocket.Chat can read it (it can take a few minutes, depending on the size of your Slack export)
Click on Import
Deselect any items you don't want to import
At the top of the page, click on Start Importing
If your Slack workspace is currently set up to hide emails, they'll also be excluded from exports. You can update your workspace settings temporarily and re-run the export in order to include emails as well.
It can be helpful to export time ranges from Slack such that the size of each import file is less than 100MB.
If facing difficulties during the upload phase of the import process, try copying the archive directly to the uploads folder using other means such as scp. Then select Server File Path as the File Type.