It's simple in principle, but it deserves some precisions and 1 or 2 tricks, because it's easy to get it wrong.
How to do it?
First of all, we agree, you have to "gild", that is to say to pass some gilding with a brush in a fine and regular layer on the top of the dough. This is important for the colouring when baking of course, but also because by doing this you will create a colour contrast between the dough, golden, and the incisions you will make afterwards, which will show the light, non golden dough. It is this contrast that will give a beautiful pattern when baking.
Then you have to draw the pattern, usually you use a small sharp knife, and that's the trick: you mustn't use the blade of the knife normally (as if you were cutting something), but upside down, that is to say you must use the back of the blade, the one that is thicker and does not cut. And that's because you don't want to split the dough, and in the process risk going through it, but just groove it with the thick side of the knife blade.
And if possible, don't hold the knife by the handle (as on this picture), because you have too much strength and you risk to crush the dough when you should be "airy", but rather hold it by the blade, between thumb and forefinger, handle up, almost like a pen.
Once you have learned this gesture, give free rein to your creativity in terms of decoration and draw the pattern of your choice: grid, spiral, flowers, grid,...
To sum up: To trace a pattern on a pastry before baking, the best way is to use the thick side of the blade of a small knife, holding it upside down, by the blade, pinched between the thumb and the index finger.
