Skills Development Levy (SDL) is a compulsory levy that you have to pay for all your employees working in Singapore, on top of CPF contribution and Foreign Worker Levy.
"All employees" means including 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 categories of employee exempted from SDL are defined here under the FAQ question : Which categories of employees or workers are exempted from SDL?
An employer is required to pay a monthly SDL for local and foreign employees even those on casual, part-time, or temporary basis.
SDL payable is at 0.25% of an employee's monthly remuneration at a minimum of $2 (less than $800 earned a month) and a maximum of $11.25 (if gross wages earned per month is $4,500 or more).
For SDL, you can pay it together with your employees' monthly CPF contributions. You can refer to this link for full information : https://www.cpf.gov.sg/employers/employerguides/employer-guides/hiring-employees/skills-development-levy-(sdl).