How should I attach a tarp to a metal shed roof?

If you know where it is leaking, cover that with gorilla tape. It lasts for years compared to regular duct tape. I had a rip in my convertible top on my car. I used duct tape and had to change it every month or two. I went to gorilla tape and it was on there for three years before the car died of old age and the inability to find a new radiator the right size.

If you have a tarp large enough to cover the whole roof, lay it on the roof then use rope at each tarp grommet and tent anchors to attach it to the ground but if you have concrete surrounding the shed I'm not sure what would be best. Roofers in our area just lay bricks on the tarp as a temporary measure

I'd go with the gorilla tape or waterproof Flex-Seal tape. Clean/sand and prep as needed per directions. Great stuff

HI Teri. You can buy big cork screw landscaping rods to put in the ground. Get good rope and tie it to the eyelets of the tarp (hoping your tarp has eyelets) then tie the other end of the rope to the landscaping rods. If you have a old stack of singles you could also put that on the roof to help hold the tarp down. Good Luck and I hope it helps.

magnets ?

I have a 10 X 12 metal shed. I purchased a 11 X 15 tarp and bungied all four corners, so far it seems to be holding quite well

Where I live many people put heavy bricks or decorative bricks because they are very heavy on top of the tarp. During a hurricane when roofs were damaged they worked great.