This is a straightforward 2 step process. First, you will export your vault(s) from 1Password and then you will use Strongbox to import them into a new Strongbox database (in the open source KeePass format). We'll try to illustrate the process below.
Step 1 - Export from 1Password
This should be very straightforward. The instructions and screen recordings below are for 1Password 7 on macOS, but they should be very similar on Windows and Linux.
a) Open 1Password and Unlock your vault with your master password
b) Click on File > Export > All Items...
c) Enter your master password again
d) Choose a location to save the exported .1pif file.
Step 2 - Import using Strongbox on macOS
a) Open Strongbox (version 1.16.2 or later) on macOS
b) Click on File > Import > Import 1Password (1Pif)...
c) Locate and select the 1Pif file that you exported in Part 1 above. Strongbox will confirm the import was successful.
d) You now need to set a master password for your Strongbox database. This can be the same as your old 1Password master password if you like.
e) Lastly, save your new Strongbox database either locally or on your favorite cloud drive.
Files & Documents
(Updated 17 January 2022)
Strongbox now supports the import of Files and Documents as 1Password have now begun exporting the documents inside the 1Pif export file. This is only the case starting with 1Password version 7.9.2 and above, so make sure you are exporting using the latest 1Password app.
Feedback is welcome, so let us know how this process goes for you and if we can improve the instructions here or the import itself.