you will get a group of objects [shapes] after the scan - to merge these you can ungroup the scan, and with all the objects still selected, press ctrl+J [join shapes] - but you must first remove the white background from the scan
so the steps would be:

ctrl+click on the white background of the scan group to select this only and delete
ctrl+U to ungroup
ctrl+J to join

depending on the mix of objects in the scan, the resulting single shape will be either black or white; if white simply recolour

to get good results it is very important to have a high resolution scan, I always scan at 600dpi

you may also need to darken the drawing first, if it is too light - and you will not get good results if the paper you have used is not scan 'friendly' and therefore does not scan with 'pure' white background

it is not possible for me to give any advice on the settings without a copy of the actual scan, but to be honest, rather than use xara for this I would suggest using inkscape, which is free; I only use xara tracer for the very straight forward scans and my starting point is:

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if I don't get good results with these settings [having tweaked the image contrast/brightness] I go to inkscape; the SVG file from inkscape will import into xara but you may need to resize...