Your spouse/partner can apply for a work permit before or after they arrive in Canada. The work permit will usually be issued for the same length of time as your study permit. Your spouse/partner does not need a job offer to apply for a work permit.
In some cases, Canadian visa offices abroad will process spouse/partner work permit applications at the same time as study permit applications. In this case, you will need to include the additional fee for the work permit and complete the work permit application for your spouse/partner, available from the website of the consulate.
Most spouse/partners come to Canada as visitors first and apply for a work permit after they are in Canada. If your spouse or partner does not require a TRV, they may be able to apply for a work permit at the border or airport where they enter Canada.