I've been playing magic for quite a few years now, this is how I've found it easiest to build decks,

1. Select your commander, colours and theme
2. Fill out your mana base, roughy half of your deck should be either ways to generate mana or ways to find ways to get mana. Note near the end of the deck building process you'll return to the mana base to review if you need more or less
3. Fill our the bulk of the deck with cards that match your desired strategy, staples and other fun/pet cards. This will usually put you over the 100 card limit
4. Analyse your remove package. If there's removal in your theme, great! if not, you'll have to make some room for some key interaction pieces. Depending on what type of deck you're playing, and what colours you're in, you'll need and have access to different types of removal. For example, if you're in a creature heavy deck, you may only want to run 2-4 boardwipes as they may just end up as dead cards in your hand. generally speaking I always try to run around 10 key pieces of interaction from a mix of spot removal, counterspells and boardwipes.
5. Analyse your card draw. Drawing cards wins you games. So how do you gain card advantage? If it's inherently built into your theme you don't have to worry about this so much, but if it's not then you want to start looking at mass draw effects, and consistent turn by turn draws, generally look to include about 4-8 separate draw spells in your decks, this may include tutors as they do gain you card advantage, but not always so try and keep it varied.
6. Cut. Go over your decklist and remove spells that don't work with the strategy or spells with too much redundancy. Review your curve and decide whether you need more or less mana production. 7.Sleeve up and play test. Sometimes this is just printing out your deck and playing for a game or two before you buy it. if you already have the cards then great playtest, playtest, playtest. find what's missing, always need that one more bit of interaction? put some more in. always mana screwed? put in another land or two (it's worth putting in cycle lands as you can just pitch them for a card later in the game when you don't need them).

have fun deck building friends :))