On each level, you get a gold medal if you finish first time, or on your first attempt since you last tried for a medal. The next attempt or two (depending on the level) will get you a silver medal, and thereafter it's worth bronze. There is a reason for this: on attempts worth gold medals, you'll get hardly any help in terms of crates. Silver will give you a bit more help, and bronze a bit more still. Thus getting a gold medal really is harder than getting a silver or bronze, in many cases. When I say below that you "get" something on a silver/bronze attempt, it means that there'll be a crate containing it.
1: Pumpkin Problems
Difficulty: 1
Go 1: Use the ninja rope to get to the right of the first barrel, then shoot the barrel with the shotgun, standing well clear. Shoot the worm to your right onto the mines.
Go 2: If you're lucky and the mines got rid of the pumpkin stalk, you can just go and grab the crate using the ninja rope. Otherwise, use the shotgun to make a hole in the stalk, then go and get the crate next go.
2: Operation Market Garden
Difficulty: 1
Use girders to get your worms into positions where they can shoot straight at the enemy, then just go for it.
3: All quiet in the library?
Difficulty: 2
Chances are you're going to be nuked in this level - you're unlikely to finish it before then. However, that in itself shouldn't be a problem. As suggested, try to get your worms away from the edge before the earthquake (which comes one go after the nuke). If you can (and you don't think the enemy will grab them), leave a health crate or two lying around so you can heal yourself after the nuke strikes.
So long as you're not stupid (i.e. don't leave worms right near the edge of the landscape, or clustered together), this level should be reasonably easy.
Go 1: Use the jetpack. Go over to the right hand side of the landscape without getting the crates that are on the "bridge" above the enemy worm. Get all the crates on the right hand side, and land just to the right of the tree. Now aim a homing missile at the middle of the bridge, and fire almost straight up with maximum power. (The power is important - too little and the missile will hit the sleight.) If you're lucky, the enemy will now slide into the water due to the flames from the exploding crates.
If he doesn't fall in the water, go over to the bridge on your second go and attack him with your other weapons... this is harder, but remember that a draw counts as a victory, so you can use the 25 points your worm can do by dying to your advantage.
5: Do the Locomotion
Difficulty: 5
Go 1: Walk and jump (using backflip) to the trunk of the tree, and blowtorch under it.
Go 2: Backflip onto the tree, collect the crate (which contains a girder), and use the girder diagonally to build a bridge between the tree and the train.
Go 3: Go across the girder, and blow the mine up by walking near it then jumping away. You should be able to do this without losing your go. Now continue to the right, dropping down onto the back of the train. Use the bungee when you need to, and you should be able to get the utility crate - you probably won't have enough time to use it though. Just get ready for the next go by climbing towards the health crate as far as possible. (It is possible to complete the mission on this go, if you're feeling dexterous.)
Go 4: (Assuming you haven't finished yet). Do or die time. Use the jetpack, and you should just have enough time to get the crate on the far right, which will complete the mission.