Apologies for re-posting this but I've just knitted one of these squares and realised I made a small mistake in the instructions. Not on the actual pattern, but the instructions that went with it. Don't panic! 
2 colour heart
This is knitted using the intarsia method, which means you twist the colours together at the back of the work when you change, rather than carrying it across. Because of this, you will need to wind your background colour (A) into three separate balls (or use plastic bobbins), which is something I should have thought of before I started knitting it in haste last night! If you make a smallish ball of colour B (the heart colour) too, you'll have less wool dangling around at the back of your work. You will also need two balls or bobbins of colour B (the heart colour).
When you join the yarns (which you'll need to do with the first ball of B on row 9, the second ball of A probably on row 10 (it won't notice if you carry it across 1 st on row 9), and the third ball of A on row 29 and the second ball of B on row 30 - as above it won't notice if you carry it across 1 st on row 29) I usually tie it on to the stitch before (ie the one on the left-hand needle that you are just about to work) using a single knot, though there may be a more scientific way of doing it. Make sure that a) you leave a long enough tail to darn it in after (you'll need a needle with an eye big enough to get the yarn through, and make sure you only weave it through the loops on the back of the knitting, so it doesn't show at the front), and b) that the tail is at the back of the work - so tie it through the back of the loop on rows 9 and 29, and through the front on rows 10 and 30.
So, you need 2 colours - A (the background) and B (the heart colour)
Knits to 15cm/6in sq using DK and 4mm/UK8 needles
K = knit, P = purl
Cast on 33 sts
Starting with a K row, work 8 rows stocking stitch (K the right side rows and P the wrong side rows)
Row 9: K16A, K1B, K16A
Row 10: P15A, P3B, P15A
Row 11: K14A, K5B, K14A
Row 12: P12A, P9B, P12A
Row 13: K11A, K11B, K11A
Row 14: P10A, P13B, P10A
Row 15: K9A, K15B, K9A
Row 16: P9A, P15B, P9A
Row 17: K8A, K17B, K8A
Row 18: P8A, P17B, P8A
Row 19: K7A, K19B, K7A
Row 20: P7A, P19B, P7A
Row 21: K6A, K21B, K6A
Row 22: P6A, P21B, P6A
Row 23: As row 21
Row 24: As row 22
Row 25: K5A, K23B, K5A
Row 26: P5A, P23B, P5A
Row 27: As row 25
Row 28: As row 26
Row 29: K5A, K11B, K1A, K11B, K5A
Row 30: P5A, P10B, P3A, P10B, P5A
Row 31: K5A, K9B, K5A, K9B, K5A
Row 32: P5A, P8B, P7A, P8B, P5A
Row 33: K6A, K6B, K9A, K6B, K6A
Row 34: P7A, P4B, P11A, P4B, P7A
Starting with a K row, work 8 rows stocking st, ending with a P row. (42 rows in all.) Cast off. Weave in all ends at back. (I usually start by putting the sewing needle through and winding the yarn around it a couple of times, which makes a knot when you pull it through and stops it from unravelling.)
As this is a stocking stitch pattern it will probably curl up alarmingly and will definitely need blocking to make it look nice and square. See GentleOtter's link on the other thread for blocking instructions (it's basically all about making it damp and pinning it to a towel or blanket with rustproof pins to the right dimensions until it dries.)