Understanding the Build
Shadow Shaman is my favorite hero and I like to push towers. The problem is he is very squishy and needs a lot of mana. This build is all about mana regen to be able to always support and knock down towers while being able to escape danger mid game.
To begin lane with a strong early game hero that will help keep you from taking too much damage since you wont have any health regen items. Get some quick farm for an early Void Stone then boots or Sage's Mask. It really only takes a few creep kills since you saved all your starting gold for void stone so your carry shouldn't get too mad. The extra mana regen early game makes a big difference, especially after gaining a few levels. This also makes wasting gold on Aracane Boots pointless.
After your three starting items you have plenty of mana regen for the time being and want to focus on a Ring of Health for Perseverance so you have some health regen. Next push straight for Boots of Travel to be able to quickly move around the map and support the rest of your team or escape to base. Depending on how the game is going you can go for Boots of Travel before Perseverance. You have probably taken down your lane's first tower at this point and will be more useful in another lane with your Mass Serpent Wards.
Once you have Boots of Travel you want to get Aghanim's Scepter. It will make taking towers fast and can be a game changer in team fights. You can get Boot of Travel after Aghanim's Scepter but its fast tho get the Boots first and they allow you to move in and out quicker and avoid dying so its best to get Boot of Travel first.
At this point you have fairly high mana regen and high damge output with your ultimate so you want to make sure you can get in and out of places fast, or at leas tget in and throw down the Serpent Wards before dying. For this a Shadow Blade is great. It'll save your life, you allow you to trap an unexpecting enemy in your Mass Serpent Wards.
If the game isn't over by now now, work towards an Octarine Orb or Refresher Orb. Refresher shouldn't take too long since you already for one Perseverance. A Refresher Orb is really only useful late game since it costs so much and uses so much mana when combined with Mass Serpent Wards. A mistake I use to make is going for it early on before or right after Aghanim's Scepter and I'd use it after Mass Serpent Wards and wouldn't have enough mana to use it again right away. Waiting until late game ensures you have more than enough mana/regen.
This build makes for quick work of the enemies towers and will greatly benefit your team in the long run, even if they complain about your lack of ward buying. Shadow Shaman is a support, but can be a very strong pusher as well with the aid of a carry. The key is staying near teammates because Shadow Shaman if squishy at all point of the game. A good Shadow Shaman takes good teammates, otherwise he becomes a feeder.
Last note. Stay away from Pugna.