In the movies, Indy's bag is a World War II British MKVII gas mask bag with the cloth strap replaced by a leather one. You can still buy these as military surplus, and there are a few prop replica makers that sell them. According to an Ebay listing for a vintage MKVII, the dimensions of the original bag are approximately 10" x 10" x 3".
This half-size bag will be 5" x 5" x 1.5", though you could also scale up the pattern to make a full size bag. It is big enough for a phone, coin purse, and a wallet, with just a little room to spare. I have left off the bottom drain vents and the round circular thing on the side of the original, but these can be replicated with grommets and the right button.
Step 1: Parts
1/2 yard canvas (or heavy linen) - washed to pre-shrink
dark brown leather (or cloth) strap - 3/4" by 65" or so, depending on your preferred length
1 inch rings - 2
gold snaps - 2
silver rivets - 2
silver square sliding buckle for 3/4" strap
paper to make the pattern
metal edge ruler
iron and ironing board/surface
sewing machine with a needle that can handle canvas (or a needle and some patience)
snap setter and hammer for snaps
green tea OR sage/ parsley/ thyme/ basil
pot that can hold 8 cups of water + fabric
old pillowcase (zippered or tied shut)
container for soaking
Note: it's ok to use food dishes for this project since you are dyeing with edible items and not using chemicals.
fabric for purse lining