Complex sentence: He originally took up art criticism when then Modern Painters (magazine) editor Karen Wright asked him to contribute to the then London based magazine in the late 1990s and in 2005 he co-founded PORT, a noted visual art blog.
Split into simpler sentences: He originally took up art criticism when then- "Modern Painters" editor Karen Wright asked him to contribute to the then-London based magazine in the late 1990s. In 2005, he co-founded PORT, a noted visual art blog.