Hi there Nina, welcome to the Guild! Well you have certainly come to the right place. I don't know of any Inkscape tutorials on antique looking maps, but there are several such tutorials here for GIMP (another freeware program). Gidde has written a particularly nice one. I'd reccomend spending some time in the Tutorials section and see if you can find a tutorial that matches the style you are going for. Look forward to seeing what you come up with.