very new/no experience, guitar build from picture, scale

hello,
i am new here and want to use this software to get dimensions of a guitar. i have a picture of the guitar and some idea of its scale but want to use this software to find out other things. i import the image ok and have looked at scaling. but that is it, no further progress. i want to make on the image two points. this will be the reference for everything else. the two points are from the nut to the bridge, this sets the scale of the guitar. the guitar i want to build is 24" 3/4 inches from nut to bridge. i want to mark this on the image which should help get everything else sorted. the idea being that once the scale is set i should be able to print out the image to the correct size. can this be done. is there a particular part in the manual as most of it seems a little confusing.
thanks
simon

edit:
i have watched a video on tracing so i have tried that. the guitar image is on a white rectangle so that doesnt help. what i have done is created a layer and drawn a rectangle from the nut to the bridge. this has a distance of 512 approx (which i believe is pixels) so i thought that with the guitar scale being 24.75 inches i would divide 512 by 24.75 to get the correct scale. does anyone follow what i am trying here? is this along the right path?
any advice would be great thanks

You are on the right path. If you are working in inches be sure units are set to inches in the current drawing settings. After having imported the picture measure the distance between your two points (nut and bridge, whatever this is) with the measuring tool. If you have set the drawing to inches, this distance will be shown in inches. So 512 / 24.75 gives the scaling factor, which you have to apply to the picture with the modify - scale command. After doing this the image will be disappeared (a bug), doing autozoom will bring it back. Now your two points should be 24.75 inches apart. A cad drawing always should show the real dimensions. Scaling down for printing is a different story.

hello,
thanks for your response.
i dont know what i am doing wrong. i clicked on tools, modify and scale and then put 20.65 in the X box which also appeared in the Y box. this has multiplied 511 by 20.65 and now the distance between nut and bridge is 10580 approx.
my image is a size that is providing 511 between the two points but i want it to be 24.75 inches between these two points. please could someone explain how to do this.
thanks
simon

Sorry, I'm not so good in maths myself. It seems the calculation should be the other way, 24.75 / 512 = 0.04834 (scaling factor).  0.04834 x 512 then results in 24.75. Have I got it right this time?