Well, I'm only a beginner as well but I'd be happy to explain my deck building process.
First, I don't have a lot of cards since I just started. I knew I wanted to mostly play Shub, but I had to mix another faction in there. I chose Syndicate because I thought their exhaustion effects might be useful against troublesome opposing characters and to get a little bit of Investigation. Originally I had more Neutral cards like Magnifying Glass to help these roles but I found after doing some test draws that getting more than one Neutral card in your starting hand can mess up your resourcing.
I decided early on that I wanted to theme the deck around Dark Young (I'm not as much into Mi-Go), and also to put Shub into the deck because of her potential to bring them back into play. Since I was planning on playing at Gencon against very strong players, I had to assume that their experience would give them a big advantage over me so Shub was going to be my ace in the hole. Even against a stronger player, if I could suddenly drop her and bring a ton of Dark Young back into play, that could give me the swing I'd need to stand up to a pro player. Based on knowing that last year's tournament was dominated by Agency/Hastur I tried to use that to plan. Many of my Shub characters already had Terror so I wasn't too worried about that, but I was concerned about getting dealt wounds by Agency so I put in Albino Goat Spawn to soak damage and a couple of copies of Dutch Courage to buff up our toughness. As it turns out, this was probably overkill, I got dealt fewer wounds than I'd anticipated.
I used the online card list at carddb to plan out what I needed. I was starting from two Core sets and a Secrets of Arkham, but was willing to buy a few boosters to build my deck and this helped me track which ones I'd need.
In order to get Shub into play in time to help me win, I'd need some accelerators. So, I picked out Priestess of Bubastis, One of the Thousand, and Eat the Dead to help ensure that if I drew her I'd be able to play her relatively soon. Also, I has a number of 3+ cost characters that could benefit from them if necessary as well.
For Events, I put in some more durability (Regeneration, Bred to Survive), and Shocking Transformation - which then led me to feel like I needed something really nasty in my deck to transform into. I wavered between Small Ghouls and Slavering Gug - ending up with Small Ghouls. I liked the Gug's abiliy but it seemed very expensive and I wasn't sure how much I'd use it. If I had to pick again today though, I'd take the Gug. Better to have an extra tool just in case.
Once I'd picked out the various cards I wanted I was close to 60 in my deck, too much! So I had to start trimming. I don't really remember what else was in there before, but most of it was done by dropping from 3 copies to 2 of things I felt I didn't need a ton of. I got it down to 52 and felt that was "close enough" for my level of skill.
So basically the plan was to flood the field with cheap tough characters, find Shub, and then bring them back again - hoping this would throw a big enough wrench into my opponent's plans to win me the game through simply having more monsters than he could deal with.
How did things work out? Well, I won one game, got close in a second (if I'd been able to hold out a bit longer until I could play Shub I might have been able to come back), and got pretty stomped in the remainder of the games. Still, 12th place isn't bad and I'm happy with the results.
One of my banes was character sacrificing abilities. I'd invested FAR too heavily in resisting wounds but had no protection against direct character sacrifice. If I were to re-tune the same deck today I'd ditch some of the toughness cards and look into what I could do to help against sacrifice or to get rid of things that cause sacrifice. Probably look into some more Events or Support cards that I could use to mess with my opponent's plans. Originally I wanted to stick with a straightforward deck but after some games I feel like I can probably use some more tricks.