Take the Infus Emerald and use it on one of the Infernals found in Desolace. After successful, bring the Infus Emerald and the Imprisoned Infernal Spirit back to Menara Voidrender in the Barrens.
So, you have chosen to capture the soul of an infernal, good.
Take this emerald--with it, you will be able to capture the creature's soul. It works very similar to spells you already know: when the creature is close to dying, use the gem and it should do the rest of the work for you.
I would suggest going to Desolace and trying your hand at any of the infernals there. Those should do nicely.
After you're successful, take both the captured spirit and your emerald to Menara.
Yes, <name>? Have you chosen? Infernal or felhound?
Each choice is personal and should reflect your strengths and weaknesses. It is not unheard of for a <class> to enhance themselves further instead of making up in areas they lack. Sometimes overpowering an enemy is just as strong a tactic than becoming a more balanced spell caster.
Ah, the spirit of an infernal. Well done, <name>.
When I have both the spirit and the cleansed orb, I will merge the two for you, but not until then.