I have also raised Swallowtail butterflies from eggs found on parsley in a fashion similar to that described here. I would be interested in hearing about the experiences of other readers in locating and identifying the eggs of other types of butterflies.
Step 1: Timeline
June 28 -- Monarch butterfly observed laying eggs on milkweed.
June 30 -- Five eggs brought inside (photos only show four).
July 2 -- Transfer eggs to a fresh leaf in anticipation of the eggs hatching.
July 3 -- Two hatched by early morning and the remaining three by noon.
July 5 -- Colored bands becoming apparent.
July 8 -- Getting bigger.
July 12 -- Lost one caterpillar. Failure to thrive.
July 13 -- Start molting period. Appear uninterested in food.
July 15 -- Finish molting. Voracious and big.
July 17 -- J-hooking.
July 18 -- Four chrysalises. Nothing left but the waiting.
July 28 -- Chrysalises darken in the evening. Wing patterns clearly visible.
July 29 -- Butterflies emerge!
Step 2: Milkweed
Milkweed has wonderful flower clusters that attract butterflies. I grow a patch outside my kitchen door. My neighbor grows a lots of flowering plants and I believe his flowers attract butterflies and then they come on over to lay their eggs!