Go to your local auto shop and get some High build primer. Lightly sand to feather the edges around the good paint and pray the HBP onto it even. when this dries rub down with light grit wet/dry paper and if you find any uneven holes or lumps respray with the HBP and repeat the rub down until its smooth.
Get the proper paint color from where ever you want I recommend may halfords or ford but your local auto shop may be fit to supply it in a tin or spray form for you just make sure the color is perfect or very close. I use a small compressor with a gravity feed gun for touch ups as i find its better to use than the sray tins but the tins are 100% fine for small touch ups. I actually used spray tins to spray 2 mirror on a mk4 Escort back in the day but paint as come a long way since then lo!
anyway when you have your colour spray over HBP in maybe 2 light coats and let dry. when dry very very lightly go over it with wet and dry paper very light grit to smooth it in and feather around the edges where it meets the good paint. This is important in order to blend it in, when you have done this and its clean and dry respray your final coat of color maybe 2 thin coats or so but you will know yourself how its drying out and if you need maybe 3 light coats or so.
Very import on this part id start from the center of the area and work out to the edge as you want very little over spray on the good paint edge as this will save a lot of hard work blending it. when this all drys it may look the part but you will need to do a coat of clear lacquer to protect it I usually put on 4 lights coats of this and using the same method as above but instead of center outwards go from left to right and try to keep it as even as you can. you can overlap the lacquer on to the good paint at the edge just a small bit say maybe a cm or so but keep this spray pattern as light and as feathered and you can.
when this all dried out leave all for a week or 2 then add on some wax ect to keep it in good form.
This is how I do paint touch ups and believe me it a very time consuming at times you will want to through in the towel but when finished it is well worth it,
Hope this helps in some way or form.