This dude definitely killed someone and has chopped up the corpse.

That said, with a rectangular tarp:

Lay out tarp. Fold two corners on the same side length-wise to the center of the tarp so it looks like a rectangle with a triangle on top. Take one bottom corner, fold to the base of the triangle. Orient yourself so you are standing at the base of the biggest triangle. Fold that triangle in half towards the base of the right/left-most smaller triangle. You should have a roughly square shape with a little piece sticking out over the side. Tuck that piece under itself and into the largest triangle (the one you just folded). You'll have something that looks like an envelope. Take the same edge the little piece was sticking over and roll it up on itself a few times. Secure the roll with rope or bungee or whatever so it can't come undone.

Congrats bitch. You have a bag now.

I make no guarantee as to my ability to give you instructions, your ability to understand them, or that your victim won't leak out the sides. It should hold blood/fluids to a certain extent though.

Are the heavy duty bags ripping or do you just not want to go out and buy them?

pre dug holes

Demolition construction waste bags. https://www.globalindustrial.com/p/reusable-all-purpose-bulk-bags-open-top-flat-bottom-1000-lbs-coated-pp-20-x-20-x-28-pkg-qty-1?infoParam.campaignId=WR&utm_source=bing&utm_medium=cpc&utm_campaign=%5BADL%5D%20%5BPLA%5D%20%5BS%20%26%20S%5D%20%7BBins,%20Totes%20%26%20Containers%7D%20(Bad)%20-%20%5BLow%20Priority%20SKUs%5D&utm_term=4583176797570897&utm_content=Storage%20%26%20Shelving%20-%20Bins,%20Totes%20%26%20Containers%20-%20(Bad)&adlclid=ADL-e5ec0e67-5249-45e1-a5da-1ff04f3808a7

Body bag

Tarps are not really leak proof.

You can by rolls of plastic film. Then tape the edges with paking tape.