Every main course in Super Mario 64 will have eight Red Coins hidden throughout it, which will spawn a star. You probably saw a star-shaped shadow in the area with the four wooden poles, which is where the star will spawn after collecting all eight Red Coins. For Bob-Omb Battlefield, the Red Coins are pretty spread out, so it will take a little bit in order to get them all. You will need the cannons unlocked for one of them, so make sure you speak with the one Bob-Omb Buddy near your starting position. At the beginning of the stage, head to the open field before turning the corner to the Chain Chomp and look on top of a rock not too far from the cannon for Red Coin #1.
You passed the Red Coin #2 on the way to the first, as it is above the moving platforms right before the Chain Chomp. Red Coin #3 will be on top of the wooden pole holding the Chain Chomp in place. Go past the wooden bridge that seesaws and into the lower open field, where you should find Red Coin #4 and Red Coin #5 by the four wooden poles. Go to the bridge leading to the mountain path, but don’t cross it just yet. On the one side, you can go down to underneath the bridge, where you should find Red Coin #6. Cross the bridge mentioned above now and look up the steep hill to spot the next Red Coin.
The problem here is that you can run up this hill, as you will simply slide right back down. So, you will need to start climbing the mountain until you get above the hill, then slide down to collect Red Coin #7. Note that sometimes the slope will be tricky and push you to the left, meaning you might miss the coin. For the final Red Coin, either continue up to the cannon on the mountain or return to the one near the Chain Chomp. Either way, launch yourself to the floating island and climb the tree for Red Coin #8. The Power Star will spawn at the four wooden poles, near where you found the fourth and fifth coins.