Send logs are automatically get logged once you create a send log DE, however for user-initiated, triggered email or email in journey need to be associated with trigger send log by checking the checkbox. If you have custom fields in sendlog DE, make sure same fields are there in your sendable DE.
Follow these steps to enable for email sends.
User-initiated or triggered email:
Email Studio>Interactions>user-initiated or triggered email>select the email you want to log into logging DE>check the checkbox next to [send logging to a data extension] option> Hit Save.
Emails in Journey builder:
Open email in journey canvas > Delivery Options > Under tracking check ‘Retain Send Log Data’.
Default fields that get stored in a send logging DE are:
JobID, ListID,BatchID, SubID, TriggeredSendID, ErrorCode.