Hi Sunny, theres more than one way to do this too. my way... is using 'non fuse', 'non woven', lightweight, see thru, extremely strong, wont rip no matter how hard you try interfacing. here its called 'traceable vilene'. cheap as chips too. last time i got some paid only $1.20 a metre. i use empty cereal boxes and sketch the shape i want on that, cut it out and use as templates. love recycling and the price is right also, lol. draw round the template on the vilene. cut it out adding an eyeballs 1/4" for the seams. sew this by machine to the right side of the fabric. if i'm doing a lot of leaves i just draw them all on one big piece of vilene as close as i can and lay it onto a big piece of fabric, then sew like a mad woman. this can then be cut out while watching tv. cut each one out, trimming the seams and any points as small as you like. then very carefully (i use my seam ripper) make a slit in the interfacing big enough to turn the piece inside out. this piece now is ready to attach to the background.
where pieces overlap on the background there is no need to sew the interfacing round the entire piece, just three sides or whatever sides will be showing on the block. those underlapped pieces can be left unsewn and turn it inside out on that side. i do try to adapt designs so there are few if any inside points and hard curves. i think they are a pita to attach cleanly to the background. sometimes i will redraw the design so i use a couple extra pieces and overlap so i dont have any inside points/curves. i fudge the design to suit my needs. heres a link with maybe a better explanation and pix.
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one will know there is interfacing under the pieces unless you tell them, its that thin. you dont need to cut out the background and remove the inderfacing like some folks do.holler if you need more info, no worrys, i do love this method heaps and happy to share my experience with it.jeanne