right

ive got new new window ledges to fit

how do i fix them so they dont move ?

Chris you have a number of options, firstly you can drill the sill/window board with a counter sunk hole and screw them followed by filler.
Option two you could bond the board in place, this depends on what you are fixing.

Timber
MDF
Pre finished board etc etc

Maybe you could tell us what you are fixing then more answers can follow.

Hope this helps.

its mdf board

Silicone with some nails if you can get a fixing. is it a timber frame home

No brick no timber and pvc windows

i hae fixed one using 18mm wpb board as supports then gripfill on that to stick the boards down

will see how well its fixed tom