There are three methods for RNAi in C. elegans. The first and easiest method is to feed the worms with bacteria expressing dsRNA against the gene to be knocked down. The second method is soaking the worm in dsRNA containing solution. The third method is directly injecting C. elegans with dsRNA. This method is very efficient and usually gives the best knockdown. However, it is also tedious and time consuming. Here is a link where you can obtain more information about RNAi in C. elegans http://www.wormbook.org/chapters/www_introreversegenetics/introreversegenetics.html#d0e257