Takes a lot of time and effort (just like I put into writing this guide!)
Basically you need to max out your Alchemy, Smithing and Enchantment.
Alchemy
Just gather ingredients. The most expensive potion I have made, and I think you can make, is Giant's Toe, Wheat and Crop Cluster. Thanks to my skills and enchantments etc I just made one of them worth 5400 gold. But obviously you'll start off a lot lower than that (maybe 700). Keep making them and the more expensive potions you make, the quicker it'll improve your Alchemy. Might be worth buying all of the ingredients from the shop in whiterun, making as many potions as you can, then wait 48 hours and buy everything else she has and repeat. Otherwise just collect stuff and keep making potions.
Smithing
Easiest one to max out. You just need a fair bit of gold (like 10k) and just go to Warmaiden's in whiterun and buy ALL his Iron Ingots and Iron Ore, then use the smelter to turn Ore into Ingots. You then need Leather Strips or Leather (converted into leather strips on the tanning thing). Can buy more Ingots from his female helper too. Wait 48 hours for the stock to refill then buy more. You need to create Iron Daggers using the smithing tool thing. Doesn't matter what you create, it all gives the same XP, but Iron Daggers only need 1 Leather Strip and 1 Iron Ingot (which are both cheap) so it makes the most sense to keep using them.
You'll need to make maybe 400 or 500 Iron Daggers before you get to level 100. Once you have them, keep them. I use the drop item and get Lydia to pick it up trick to make her carry them all.
Enchantment
For this you need plenty of soul gems. Also go to the first stone you come across at the start of the game (just south of Riverwood) and get the Mage blessing or whatever it is. That improves magic skill learn-time by 20% so it'll go slightly quicker.
Next head into Dragonsreach and to the room on the right with the bloke in the black robes. Buy ALL of the soul gems he has (if you have the cash, otherwise just buy the empty ones and go out and fill them on Mudcrabs or something).
You'll also need to have some items which you can use to learn enchantments if you haven't done so already. Stuff like a sword of flames or whatever (although if you can find something with Paralysis and destroy it to learn that skill, enchanting items with Paralysis makes items more valuable).
Then get your Iron Daggers back from Lydia and just keep creating Enchanted Iron Daggers (don't worry about the soul gem you use, I believe just the fact you enchanted something does the same XP). You can then sell your daggers to the bloke in the black robes, wait 48 hours, buy his new soul gems and repeat. You'll probably have to do this for like 5 hours to get to 100.
PERKS (IMPORTANT)
Don't waste your perks on stupid stuff like two handed etc. You NEED something like 20 of them to ensure you can make the best weapons and armor in the game.
You want 5 perks in the first Alchemy, Smithing and Enchantment perks (effectively doubling their skill) and I'll have to confirm the rest you need later.
Maxed Out
Once you've reached 100 for the above mentioned 3 skills, you need to do a loop of creating enchanted Alchemy equipment, using that to make better Enchantment and Smithing potions, then using those potions to make even more powerful enchanted Alchemy equipment and repeat as long as you like.
The equipment you can make to improve Alchemy includes: Falmer Helmet (See Boost Glitch below), Circlet, Gloves/Gauntlets, Necklace, Ring.
Clothing to improve Smithing includes: Armor/Clothes, Gloves/Gauntlets, Necklace, Ring
Then once you've created a potion that multiplies the Enchanting to something stupid like 37%, create Smithing gear (to go along with your fortify smithing potion) and you should be able to improve your weapons by around 200%. Just remember to take the potion right before you improve something.
I'll update this with more details when I'm actually on the game, but that's the basics.
Boost Glitch
When you're out on your travels, make sure you collect as many Falmer Helmets as possible. There's a glitch in the game that lets you wear a Falmer Helmet AND a Circlet at the same time, meaning you can wear 5 Enchanted items which improve Alchemy instead of 4.
Err, like I said, I'll update it but that's about all there is to it. With enough time and effort (I spent probably 20 hours just improving skills, enchanting and making potions to get to this point) you can make weapons and armor that improve everything to insane levels. My bow does over 400 damage when used with my apparel which improves bow damage, and my armor is improved to like 500+.