This section will tell you how to do what on where, whom, and when.
But you have to know why first.
Let's say I'm up against a QoP in the laning stage. What spells do I choose? Think of what you need.
Let's say QoP is harrassing me badly with Poison, and I'm out of tangoes. Obviously going back to base to regen or getting a salve from the courier wastes too much time, so you should predict this. Either you get points in Quas early, or your get extra regen items if you're getting Exort early. This is only an example as you should learn to pick what spells/orbs.
But the main point is you shouldn't be restricted to a build. Be Creative. (green is not a creative color)
Why go for Q-W build when you can go for Q-E? W-E? Each specific build has it's own purpose and effect.
Q-W (GottaGoFast) gives you mobility, utility. It's great against other mid heroes and supports. You're like a Puck. You disable, you jump around, you annoy people. And eventually people die.
- Mobility, while other heroes have blink, or get Dagger as a core item, you have Wex.
- Mass disables
- You don't really have damage.
Q-E (Hit-Hard) is the middle, the balanced build. Suitable for most games, and overall is the safest. You still have disables, you still have damage, and you're still strong as ever.
- Insane Damage
- Pushing Power
W-E (HarderBetterFasterStronger)(Burst) are more damage oriented builds, they're risky, but are great if you're successful.
- Low Mobility
- Low Survivability
Let's say you're up against a team like [QueenofPain, Windrunner, PhantomLancer, KotL, Bane]. Obviously this is a very elusive team and is very hard to kill. That is, if you went Q-E or W-E build. Q-W will help you best in this build just because Tornado/EMP/Coldsnap is just more effective against a mobile team than spell that are hard to land (Meteor, Sunstrike), or spells that require you to hit them (ForgeSpirits, Alacrity).
Another example is where you're up against a team like [Pudge, Clockwerk, Na'ix, Jakiro, SD]. This is more of a tanky line-up, and are resistant to magic based spells. They are weak against minus armor and physical attacks. So Q-E is the best build in this case.
And lastly, if you're against a lineup like [Storm, DarkSeer, Void, Enchantress, Dazzle]. Heroes that are almost impossible to kill, is when you should be using W-E build (both builds).
These are only examples and you have to decide on which build you're going to choose. The mid hero has the most impact in the game, so that basically means you will choose how the game goes depending on your choice of build.
Who do you target in a fight? You need to prioritize who is more important to their team. In most cases it usually is the hero you went against in the laning stage, their mid laner. Their mid laner initiates the fight, and usually has the most impact in the game and in fights. Try to target them first. Then the supports, because they're job is to be useful as possible, and if you don't kill them they usually do theirs. And lastly the carry. Because most of the time it's usually your carry who kills theirs so you don't have to worry about it, and besides a carry usually is the tankiest of the team, and trying to kill the tankiest is kinda stupid. Simply overpowering and outnumbering the carry by taking out the squishies first is much more simpler and effective.
Sometimes you go a build against a team but they have 1 or 2 heroes that are resistant to your build. For example you when a Q-W build and they have a Na'ix, or you went Q-E and they have a Storm, or you went W-E and they have a Pudge. How do you deal with these heroes? Think. Think of what their weaknesses are. In the Na'ix situation, if you want to kill him, you have to initiate him, then he will use his spells, simply kite him around until his spells run out and then use your spells. In the Storm situation, you're either going to have to have a support with a disable with you, or you need to build Orchid/Hex yourself. In the Pudge situation, you're either going to have to be really careful and make sure there's nobody else to help him, or reconsider completely. Because once you use your combo it's over. You have nothing, no escapes, no spells nothing.
You have to think of your opponents weakness and emphasise on it.
Where are you right now? Where is your position in the game? Is your team ahead, or is their team ahead? Let's say they're ahead and killing you in teamfights. You have to be careful because they're on the offensive and you're on the defensive. Don't get caught, help your teammates, protect your carry, and farm to even the odds.
Farm = xp, gold. xp=level advantage, gold=item advatange.
Knowing where to b and where to fight is essential.
TL;DR you have to create your own Invoker. Don't just do whatever a guide says (what?) and do things that you think is helpful to your game. What you want, and what you need are different things, remember that.