All game long you go full aggressive. When you are both farming, observe your opponent closely. Always have an eye on him.
If he reacts different you can assume that the junbgler is near. Then it's your turn to react. If you feel confident about the situation you can engage the 2v1.
But mostly you should just stay defensive. Try only to lasthit and be near your tower.
After you noticed that his behavior is like before you can be sure it's a 1v1 again.
Scare and pressure your opponent as much as possible.
It's your job as an assassin to be feared.
Ask for 2-3 Ganks. Kill your enemy top-lane a few times so they'll focus you.
When you reach this point, you must be careful. Put wards up.
As much as possible. You want to be sure none is there if you engage or if you're far away from your tower.
Show your enemies that you're there whenever they want to do anything.
Also maybe try to steal the enemy red buff. (when playing on the blue side)
or stealing the blue buff. (when playing on the purple/red side)
Warding is explained in the 'warding' chapter below.
Your engagement is a simple rotation you should be able to learn pretty quickly.
If you engage:
If you are beeing engaged on:
As you can see it's not that difficult. Keep in mind that your Burst of Speed resets your auto attack!