Start by laying out your blocked scarf nice and flat, stretching out the sides so the lace appears as you want it to end up looking. Then fold over the edges of your fabric on all sides (I started with the long sides and folded the short ends after) and press with a hot iron. Since I used a lightweight cotton shirting, I could set my iron on the highest setting and steam the hems for extra crisp edges. If you use a different fabric make sure you know what type it is, cotton and linen can withstand high temps, but wool and silk are prone to burning and anything synthetic could melt at high temps. (You can use a knit too, like cotton or wool jersey and you don't even have to fold over the edge! Just cut the strip the same width and length of your scarf!)
To whip stitch, insert your needle in between the knitted part and the fabric at one end (so your knot doesn't show on the outside) then come up through the fabric. Catch the strand of yarn directly opposite where you just came up, insert the needle back into the fabric and poke it up about a quarter to half inch down the top of the fabric in one stitch, so you hardly see any of the thread.