Hi Gurus

I'm new to sed.
How does one replace / with // for a string for example
//filer/test/test2/test3/test4 to become ////filer//test/t/test2//
test3//test4 ?
I have tried a variety like
TEMP_VAR=`/usr/bin/sed -e 's/\//K/g' //filer/test/test2/test3/test4 `
and I still can't get it right.
I'm not sure how to escape /.

Any is appreciated.