Test your teflon hobby iron heat setting on mylar corner scraps. Too low and the seams pulls apart easily. Too high and the mylar starts to look kinda melted. Avoid touching areas to be heatsealed.
Thicker seams are heavier but hold gas better.
Masking tape holds things down. Short carpet is good to work on with a piece of smooth cardboard moved along cutting or ironing line. Visualize tiny helium atoms flowing freely thru seam flaws. Small deviations in ironing lines appear as bumps and wrinkles in blimp. Nipples can be cut from a small light weight plastic bottles.