I got the "Sworn to Darkness" Commander 2014 deck, and immediately began to edit it to suit my needs and blending my extensive vault of black cards. First off, I had to toss all the cards I felt were overcost for their effect and cards I had more efficient alternatives. I cut the land count, but added artifacts, creatures and instants to replace them (the deck came with 40 land, which is a 2:5 ratio, which is higher than I am comfortable playing with.
As I continued to examine pros and cons, I continued to cut cards, some were cards I added that I thought would be good, others were cards from the original deck. Many of these cards are excellent cards, and I had to search for a reason not to include the card. Some examples: Tragic Slip, costs only 1 black mana (pro), used to destroy a creature (ideally), but with so many multiple creature destroyers out there, taking out one threat seemed like underkill; Black Sun's Zenith, put -1 counters on all creatures then reshuffle it back into the library...sounds great on paper, in a normal 60 card deck, it would be awesome because it would re-emerge so many times...but in a 100 card deck, you may only cast it once; AEther Snap, is a one-time trap (basically), turn attacking creatures into 0/2 non-threats and slaughter them, or turn defending blockers into sacrificial lambs when you attack...but you only get one use of it, so it'll probably just sit in your hand the whole game as you wait for the right time to use it. That's the process I use in whittling down the deck to a legal, playable size.
Some cards I just decided to "get out of the way", since this is a creature-heavy deck that needs a large creature body in the graveyard, cards that exile creatures are counter-productive, cards that stray too far from the central strategy are also on the chopping block.
Another central consideration is that this is a LARGE creature-centric deck, with a huge number of 5+ mana casting cost roster of demons. If I have any chance of getting these nasty creatures into play, managing my mana sources becomes the first priority. Using artifact, spell and creature ability mana sources will be CRUCIAL to get those 7, 8, 9, 10 mana cost creatures into play in a timely manner.
Second to mana management, is draws. The cards included in the store-bought version of the deck addressed this issue, but I found their solutions to be substandard, so I hunted for some more economic card drawing sources. Some of the potential draw sources I found, I later declined to use for different reasons.
My version of the deck focuses on the DEMON creatures, although I need enough sacrificial fodder to keep those nasty demons in play, so there are still a few zombie/horror/demon token generators to create a suitable sacrificial horde, though I know I'll need to sacrifice some of the big baddies to their brethren at some point.
Early in the game, focus on getting MORE mana into play (the assortment of artifacts especially), as there are very few low cost creatures, and not that many medium sized ones that you will want to use prematurely. The opening hand should have at least 3 land, any less would be suicidal. If you don't have 4, then at least have one of those 1 or 2 cost artifacts which can offset the handicap.
Aside from that, get what you can into play as soon as you can. The deck has a plethora of creature destroying/life gaining spells which should both remove threats AND offset the life sucking upkeeps of the larger creatures.