Here's a random guide from the internet:
http://sewing.about.com/od/techniques/p/gathers.htmyou can google for "attaching a ruffle" to see other guides. Some even have videos, so it is really helpful.
And this is my little guide. Disclaimer: I haven't actually done a ruffle myself before apart from gathering and attaching lace. These are just the steps I would follow. Feel free to add/correct/ask questions.
This suits fabric that doesn't show holes if you are planning on having an over-fabric or edge of fabric ruffle.
1) Choose how much you want your ruffle to be gathered. 1.5 times for a looser look, or 2 times for a tightly ruffled look, for instance. How much you can gather depends on the thickness of your fabric. Calculate the dimensions of your ruffle, adding allowances for seams and gathering. Cut from your fabric, joining pieces as necessary. It would be even better if you planned the seams in the ruffle to be at unobtrusive places like matching other seams. If you wanted your ruffle to be two fabric thicknesses you can do the whole thing very neatly by sewing tubes of fabric right sides together, turning it the right way, choosing where you want the seam to lie, and pressing to form a flat piece of fabric. With the tube method you can skip the neatening step because all the raw edges will be enclosed. The tube method would be most useful on the over fabric ruffle.
2) For making a ruffle over the fabric, you would neaten both the long edges of the ruffle before gathering it. You might even want to fold the top part so the edge falls below the sewing line, so you don't see any edges. For making a ruffle under the main fabric, you only have to finish the hem before gathering.
3) Make two rows of gathering stitches on either side of the line you want to sew. For an over fabric ruffle like the bottom of the skirt in the first picture it would be a few cm (an inch or so) down from the finished top, depending what you want it to look like. For an under fabric ruffle you would just do gathering stitches 6mm or so either side of your normal seamline.
4) Pull the gathering threads (one from each line of sewing) from your sewing and distribute fullness evenly. Check that the ruffle fits the hem of your skirt (or whatever you're applying it to) and gather or un-gather until it does.
5) Attach your ruffle to your garment by sewing in between the gathering stitches. For an over fabric ruffle, apply the wrong side of the ruffle to the right side of the garment. For an under fabric ruffle, sew with right sides together (if you want it to look like it is coming straight from an edge of the garment like the floral ruffle on the last dress). You'll want it to be a sturdy stitch so you can remove the lines of gathering stitches afterwards (if an above fabric ruffle). If the gathering stitches cannot be seen (as in some under fabric ruffles) just tidy up the thread ends and leave them in place.
Edit:
I would recommend doing everything (apart from hemming and attaching the ruffle if you don't want to) by machine. This deals with a LOT of fabric, since you'll probably be gathering the ruffle 1.5 or two times its finished length. A ruffle that comes off the edge of your garment, like the floral ruffle in the last dress won't show your stitches, so I see no aesthetic reason to do that kind by hand. The other kinds of ruffles would show stitching, unless you are attaching a ruffle that is completely under your garment to a different layer (like lining, for example).
Out of interest, why don't you like using your sewing machine?
