Hi, GaMommy2,
MANUFACTURER'S DIRECTIONS: The fusible interfacing has two sides - one smooth and one not-smooth. The not-smooth side has the heat-activated adhesive that is fused (ironed) to the wrong side of the garment fabric. Typically, the iron is set at medium high without steam; any higher temp and the adhesive could pretty much dissolve away. It's PRESSED, not ironed and there's a difference. When you press, simply place the iron on the fabric and let it set a few seconds to do its job, then pick it straight up and place it down in another spot. With fusible interfacing, I typically count toa moderately fast 10 before picking up and moving the iron. (When you iron, you rarely pick it up. This can cause fusibles to shift as the adhesive melts into the fabric and you end up with a warped garment piece.) Before fusing the interfacing to the fabric, though, take some scrap pieces of the interfacing and the fabric and experiment to find the timing and technique that works best for you and your project.
As for your SECTION dilemma, I can empathize. I pretty much hate most pattern instructions because they are not as well written as the manufacturers think, and current construction takes shortcuts that produce inferior finishing. Also, many pattern lines do not originate in the US and not-excellent translations occur. ('Nuff said.)
Look at the pattern layout diagram, which should have a layout diagram for the interfacing. If it does have such a diagram, then there are pattern pieces specifically marked "interfacing." If not, look through all the pattern pieces and pull out the ones that say "facing" (front neck facing, back neck facing, front facing, back facing). With the smooth side of the fusible interfacing up, place the interfacing fabric on the "facing" pattern pieces (also right-sideup) and cut identical pieces; then, with the smooth side of the fusible interfacing up, place the interfacing fabric on the wrong side of each facing fabric piece and fuse (iron) together.
Other places where fusible interfacing is typically used are cuffs, waistbands, collars, center fronts where there are buttons, and necklines. Does the dress have any of these components that are NOT addressed above?
I'll stop here because I am not sure if this answers your question or not and I'd rather not add further confusion.
If this is helpful but you need more help, email me directly with the name and number of the pattern and I will do my best to find it and figure it out from another angle. Very often the design dictates where and how interfacing is used.
Please don't get discouraged - it's not your fault THEY are unclear. Good luck, GaMommy2 !