Look. People always love pastry, but let me tell you this. People will love you, much, much more if you make the pastry yourself. Shortcrust is easy, and I reckon that being able to make the stuff by hand is an excellent skill to have in the kitchen, one worth acquiring for the way it makes people feel about you if nothing else.
Two things: butter must be fridge-cold, and, more importantly, the less you touch it all the better. Cold cold cold.
Rub 40z of butter and 6oz of flour together in a big bowl. Do so quickly, as Tamasin Day-Lewis says, "as though the butter and flour are hot sand". She is right on - it's exactly the action you need. When it looks like chunky breadcrumbs, add a tablespoon of iced water (with cubes from the freezer). Mix with a spoon. Add more water, but more water equals shrinkage so less, obviously, is more. Do it s-l-o-w-l-y. Do it carefully. Things should be just coming together. Make into a ball, cover and refrigerate for 30 mins. Good for tarts both big and small.
To make jam tarts preheat your oven to 180 C, putting a tray in to heat up at the same time. Roll out the pastry*. Butter a muffin/cupcake/gem-scone tin. Cut circles of pastry using the rim of tumbler or whatever will allow a generous overhang on your chosen tin. Dollop heaped teaspoons of jam into the hollows. Tamasin suggests raspberry or apricot are best and she, whose idea this is in the first place, is, yet again, right on. When the oven's ready, put the tarts in on top of the tray and bake for about 15-20 minutes. They are gorgeous.
*rolling pastry out in an even shape can be a tricky job. Delia describes the action perfectly (point 8 - scroll down). Patience, as with most things, is key, and that quarter turn stuff really does work. These days I look upon it as a little moment to enjoy using my hands, free from typing and such. Pastry, like bread, is a bit like craft.