My process for evaluating any such procedures is to consider how likely i am to end up in A&E, with a Doctor struggling to conceal a grin and asking "So how exactly did this happen" ?
So deff not a blade or scissors ....Oh no,no,no.......if you really have to do this then:
1) Go out n pay for a back, crack and sack.
2) If you have to be a gurl and use an electric trimmer, just be sure to do it properly and practice some imaginative topiary.
3) Man up and use a strip of Duck Tape.
Of course i am not being serious, this is just a cheap laff being handed on a plate to me.