Teaching complex sentences - how do you do it?

I am new to year 6. In the first week my class wrote a story extension form a visual story starter. When assessing, it was obvious that these children will not get above a level 3a if they weren't using complex sentences. I've researched this, but would like some more advice and guidence.

Please help. Thank you in advance.

 

I hadn't seen that before, The neverending sentence made me laugh.