Rouleau loops have a reputation for being really difficult, and yes they are a little bit fiddly, but as long as you have some patience, you should be able to do it without too much stress.
I use the old-school needle and thread version to turn mine, but if you have a fancy loop turner thing, go ahead and use that. I'm demonstrating the old school version here because I don't believe a loop turner is an absolute necessity.
The first thing to do is fold your bias strips in half lengthways and sew a seam down the middle. When you come to the edge, turn your strip so the seam goes off the edge (the cut edges, not the folded edge). This makes a funnel shape which makes for easier turning, I've found.
Important: leave really long thread tails. Like, the whole length of the strip, long. You need them for the next step.
Here's a closeup of the funnel shape you'll create.
Trim the funnel like so (below), right down to where the seam meets the cut edges of the bias strip. Again, I find it helps with turning if you cut it at a bit of an angle.
Next, take a bodkin and thread one of the thread tails through it, and then knot the ends of both thread tails together securely, so the bodkin is suspended on a loop of thread. I haven't got a bodkin so I'm using a darning needle.
Poke your bodkin or needle into the funnel and down inside the fabric tube.
Continue to work the needle/bodkin down the tube using your fingers. Be gentle; you don't want to pierce the fabric at any point.
When you reach the other end of the tube, pull out the needle/bodkin and pull the thread through.
Now, with the funnel end of the tube in your left hand, and the hanging thread tails in your right (or vice versa, if you're a lefty), pull gently on the thread tails and the end should start to fold into the funnel.
Keep pulling gently, don't hold the tube too tightly, and the end of the funnel should fold in on itself.
Once you've got it started, you can keep gently pulling until the tube folds in on itself. Because it's cut on the bias, the girth should stretch enough to accommodate the seam allowance, even though they're almost the same width. Careful not to pull too hard, or the thread will snap. And that is difficult to recover from.
Pull gently until the funnel end re-emerges.
Now you can take hold of the funnel end and pull that gently through until the whole tube is the right way out. Ta-da!
Once you've made your rouleau loops, it's time to attach them to the dress. I would attach them at the front first, over the seam in the lace. Leave 2-3cm overhanging the dress fabric and place that overlap over the seam that joins the lace to the dress. This helps it stay secure, as you're not depending on the strength of the lace alone. Sew it down with a small, narrow zig zag.
From the right side, you'll see the strap through the lace, but it shouldn't be terribly visible where the end is anchored to the dress fabric.
For the back straps, try the dress on to measure the strap length and get the position right, then sew them on with a zig zag in the same way, anchoring 2-3cm of the strap on the fabric/lace seam.
I think we've covered the most difficult parts of the dress construction. Well done if you've made it this far! Leave me a comment if you want to ask a question about any aspect of construction, and do tweet me or send pictures of your finished dresses - I love to see your makes!