I've done a lot of machine raw edge applique. I used Wonder Under for years and years. Then one time I ran out and really needed to finish something and grabbed Heat n Bond lite. I'll never go back to WU! I don't get nearly as many edge frays with the HnB as I did with the WU.
You can start with other people's patterns for wall hangings and applique. There are thousands of patterns for wall hangings. I still do some wall hangings from patterns.
I design a lot of my own now too. I generally start with a size that I want to work within. Right now I'm designing two series, one that's 8.5 x 24 and one that the maximum width is 36 inches so it fits on my decorative hanging rod.
You can use several different stitches to get the look you want. I do a lot of straight stitching with invisible thread. I also use the blanket stitch with matching or contrasting thread depending on the look I want. I've used variegated thread with a straight stitch. A black blanket stitch can look very folk arty.
Art quilts are all about your own creative decisions.