Step 1: Sign into your gandi.net account
Step 2: Select your domain (click on the Manage tab if you don't see a similar screen after logging in)
Step 3: At the bottom of the screen (under Technical Settings), click "Manage DNS zone file"
Step 4:

Remove the unneeded entries as indicated in the screenshot (i.e. everything except the "A" record and the www CNAME)
Step 5: Edit the A record and change the value to 65.39.205.54
Step 6: Edit the CNAME record and change the value to yourdomainname.com. i.e. domain dot top-level domain dot; the last dot is important!
Step 7: Click "Submit changes". Note that it may take some time (up to a day) for this change to take effect.
Step 8: Log into your squarespace.com account.
Step 9: Visit your Website Management console
Step 10: Click "Custom Domain"
Step 11:

Enter your domain name without a www (and without a trailing dot). It will ask if you also want to create the www - say yes.
That's it! If you don't see any results right away, wait a few hours. DNS changes have to get copied all around the internet once they're made, and can take up to a day to do so.