(1) The answer could be find in any good textbook or book chapter related to radiometry and photometry (example:
A.M. Pravilov: „Radiometry in Modern Scientific Experiments“, Springer, Wien, NewYork, 2011).
(2) The accurate conversion is only possible if radiation source relative spectral characteristic is known, otherwise is possible to derive only rough estimation using constant ( 1 W = 683 lm (for 550nm) ) and basic photometric unit definition formula.
(3) Always when one need to study some optical phenomena the good understanding of terminology related to quantities used is very important.
(4) There is no fast and good answer applicable without good knowledge environment related to the topic.