"All employees" includes Singapore Citizens, Singapore Permanent Residents and Foreigners of all Singapore work pass types, including full-time, casual, part-time, temporary type of contracts, except the exempted employee categories.
The exempted employee categories are defined here under the FAQ question: Which categories of employees or workers are exempted from SDL?
An employer is required to pay the SDL monthly even for those on a casual, part-time, or temporary basis.
The SDL payable is at 0.25% of an employee's monthly total wages at a minimum of $2 (less than $800 earned a month) and a maximum of $11.25 (if total wages earned per month is more than $4,500).
You can pay SDL together with your CPF contributions. Refer to this link for more information: https://www.cpf.gov.sg/employer/employer-obligations/skills-development-levy