Figure out your gauge per inch. Take a piece of graph paper and draw a square that matches your gauge (it won’t be an actual 8 x 8 square but it will have the correct number of “stitches” in it for your pattern.) Each small box inside your 8 x 8 square is now a stitch.
Now draw (or trace or have someone help you if you aren’t artistic in that way) a heart about the size you would like to see it in the center of your square. You might be able to find a template for a heart pattern that you can copy and size up or down in a copy machine. Make an x in pencil in each of the small squares that fall inside the heart.
You can now use that as your chart for your square. Outside of the heart area could be a plain knit and the heart could either be all purls or you could get fancy (you’d have to play with the squares a bit) and map out a seed stitch pattern for the heart.
I did a search and found a couple of links for patterns that you might be able to translate into a square:
Knit and Purls hearts