Tier 2 doing acting/modeling jobs once in a while

The contracts on acting/modelling are not employment contracts and the artists are always required to take care about own taxes. How can a person with a Tier 2 (employment) visa combine these with a full-time job (when the full-time job is not affected by acting work)? and how should the taxes be paid in this case? Thank you.

By registering with HMRC as self-employed and submitting an annual self-assessment declaring both employed and self-employed earnings. There is a legal requirement for you to register within 3 months of commencing self-employment.