FediTips has a great write up here:
We’ll do an abbreviated version assuming you’re moving over to a new account here on CoSocial.
Log into your CoSocial account at https://cosocial.ca, go to Preferences > Account > Moving From A Different Account (at the bottom of the screen), click on Create an account alias and follow the instructions.
log into your OLD account on the old server’s website, go to Preferences > Account > Move To A Different Account (also at the bottom of the screen), click on Configure it here and follow the instructions.
This will start transferring your followers to the new account. The accounts won’t transfer all in one go, they will come across in waves depending on how busy their own servers are. Some of your followers will automatically follow your new account straight away, while others may take hours (or in extreme cases even days!). Don’t worry though, you can carry on with the next steps while this is happening in the background.
Go to Preferences > Import And Export > Data Export and download all the CSV files one file at a time by clicking on the CSV icons.
On your CoSocial account, go to Preferences > Import And Export > Import and upload the CSV files one at a time (the files which you just downloaded in the previous step). Remember to select the correct file type from the drop-down menu for each CSV file before you upload it.
Your Followers (those who followed your old account) will move automatically as part of the process above.
Your Follows — those accounts that YOU follow — need to be manually imported with this CSV file. You can bring over all your exports — but this Follows one is most crucial.