If you boinc directly go to alpha sparc network portal rwgister yourself. Thats done by doing some compute work for rosetta and then adding a specific secret message in a thread you get verified in 24 hours. Once verified you need to all eligible projects userid value in the next screen. Once done every wednesday they look at your contribution and calculate sparc for you. You can withdraw using erc20 withdrawal address.
Hope this helps
Not in front of system hence no links listed.
In my case SPARC was collected automatically, like Gridcoins..
I just ran and still run Boinc on 2 my computers and SPARC is added, probably once a week as you mention.