Split stitch heart - I love embroidery, the control freak in me loves trying to keep my stitches nice and neat, this was a fun little project to work on split stitches, which are really easy. For thicker lines, just use more strands of thread, but use an even number, since you will be splitting each stitch in half. For this project I used two strands of embroidery thread.
Pull the threaded needle up through A. Stitch down through B (the closer you are to A, the thicker your line). Come up through C, in the middle of the stitch you just made, splitting the threads in half.
Come down through D, trying to make it the same distance as A was from B. Repeat and you'll create your line - split stitches make a nice outline in patterns. Make your line thicker by using more threads or by keeping the distance shorter, which makes your line seem fatter.
If any steps are confusing, let me know! I'm a beginner myself, so if you see any inaccuracies, let me know too!