I have been searching for a solution to move twentyten sidebar to the left.

All the help I found created issues with floats and widths etc.
but this code worked with me.
but I also had changed the width to 980 px.

/* LAYOUT: Two columns-reversed
DESCRIPTION: Two-column fixed layout with one sidebar LEFT of content */
#container {
float:right;
width: 100%;
}
#primary, #secondary {
float:left;
clear:left;
}

I just added this at the end of my child style.css
I’m no expert but I think this may work a lot of the time because it doesn’t alter any widths etc.