Ok

To open a second file (the parchment) in your main map, open your main map first, then just click "File/Open as layers".

Pick a decent sized parchment file (bigger than the map), and when you have imported it right click the new parchment layer in the layer stack and pick "Layer to image size". This will trim the edges of the parchment to exactly fit the map.

Scaling smaller pieces of parchment up to fit your map is definitely to be avoided, since this tends to lead to all kinds of problems with pixilation, which is where the parchment starts to look fuzzy and blocky compared to the rest of the map. Its practically incurable, so best not go down that road