A chart is a plan: each square says where to put which thread. Once four or five conventions make sense, every chart looks the same.
This is the question that stops most people before they have started, and it has a short answer. A cross-stitch chart is neither a drawing to copy freehand nor a score to decipher: it is a grid where one square is one stitch, and a symbol or a colour tells you which. The rest is counting.
The single rule everything follows from. Each little square on the chart is one cross on the fabric, and there is no scale to convert: a chart 60 squares wide is 60 stitches wide, whatever size paper it was printed on. An empty square is left empty: that is bare fabric, not a mistake.
Many charts show a symbol in each square rather than a colour. That is not fussiness: two neighbouring greys are indistinguishable in print, and a chart in symbols stays readable photocopied in black and white, which is what half of all stitchers do so they can cross squares off in pencil. Charts from here are in colour, with an optional keyline that redraws the silhouette, useful precisely when two shades are too close to tell apart.
A heavier line every ten squares. It means nothing for the stitching: it is there only so you can count in tens rather than one at a time. The same thing is done on the fabric with a tacking thread run every ten threads, and it is the most profitable advice on this page: ten minutes of tacking at the start saves an evening of unpicking later.
Charts mark their centre, usually with two arrows on the edges. You fold the fabric in four to find its own, and start there. The reason is dull and sufficient: begin in a corner, drift by one stitch, and the motif runs off the fabric. From the middle, an error has somewhere to go on both sides.
This is the shopping list. Each line gives the thread's reference and, on charts from here, how many stitches of that colour there are. The number is what matters, not the name: "310" is sold everywhere, "Black" means nothing at a counter. That number tells you what to buy: one skein of stranded cotton covers about 1,500 stitches at two strands on 14-count.
Always count from something already stitched, never from the edge of the fabric. A stitch in place is a certainty; the edge is twenty centimetres and three possible mistakes away. Work colour by colour across large areas and area by area on detail. Keep a pencil to cross things off as you go, because finding your place again is the real cost of a break.
Charts published elsewhere sometimes use half stitches, quarter stitches, or an outline worked in backstitch over the finished motif. Those are a designer's refinements. Charts from here contain none of them: they are whole stitches, one square one stitch, which makes them easier to read and a little bolder to look at.
It costs nothing, it takes a minute, and the chart is yours.