plan your pixels. You could use a computer if you want to get fancy.

cut out all your felt into uniform squares. (this is important otherwise they won't fit), remember to cut two times the squares for the front and back, and one times for the side.

start sewing squares together, keep them laid out so you know what goes where, it can get confusing.
I hand-sewed the squares because they were small and fiddly, this part could go much faster on a sewing machine. I used 3x3 (centimetre) squares the total heart turned out about 22cm's (8.7 inches).

(once you have both faces sewn up, join them together inside out, leaving a small 2-3 square opening unsewn)

it will look something like this.

turn it inside out...and stuff it.
Then sew up the remaining gap in your neatest most invisible stitch.

Ta-daaa... have a break and admire your hard work.