I like tribal decks and dual colored decks. First I decide on a tribe. Then I go to scryfall.com and search t:legendary t:(tribe) o:(tribe) and look through all the commander candidates that are 2 colored. Once I have a suitable commander for the tribal deck. I go to edhrec.com and look at decks people are playing to gauge what kind of cards are good for the deck. Then I netdeck a basic tribal deck with the commander from edhrec. Then I refine the deck with scryfall searches to find better cards for the deck.
I use these rules for the deck (slightly different rules for landfall decks).
37 <= Total Lands <= 40
19 <= Total Basic Lands <= 40
0 <= Total Tap Land <= 5
0 <= Total Colorless Land <= 5
1 <= Total Destruction Land <= 3 (I hate
Gaea's Cradle and
Cabal Coffers)
10<= Total Ramps <= 15
5 <= Total Draw Cards <= 10
1 <= Total Tutors <= 2 (I think tutors that can search any card should be banned in commander - some decks become auto win with tutors)
29 <= Total Creatures <= 32
20 <= Total Tribal Creatures <= 32
2 <= Total Artifact/Enchantment Removal <= 4
2 <= Total Boardwipe/Creature Removal <= 4
0 <= Total Counters (for blue decks) <= 4
100 >= Total Cards that fit the theme of the deck >= 20
I try to keep the mana curve average at around 3.5. 3.4 is best but I sometimes get to around 3.75.
You want the mana curve to peak at 3 and the bars to be descending on both side of 3. 0<1<2<3>4>5>6>7>8>9
For mana source distribution (deciding how many lands of each color). I look at X (Major Color's Mana Distribution) and Y (Minor Color's Mana Distribution)
Then I plug those numbers into the formula. (For more then 2 color decks, the formula is slightly different)
if Y/(X+Y) > 16.666...%
y (number of minor color lands) = (#Total Lands - #Colorless Lands - #Multicolored Lands)*(1/2)*(1/2 + (Y/(X+Y)))
x (number of major color lands) = (#Total Lands - #Colorless Lands - #Multicolored Lands) - y
if Y/(X+Y) < 16.666...%
y (number of minor color lands) = (#Total Lands - #Colorless Lands - #Multicolored Lands)*(Y/(X+Y)*2
x (number of major color lands) = (#Total Lands - #Colorless Lands - #Multicolored Lands) - y
I use mtgassist.com to find cards that are similar to each other. For example if you search
Propaganda in mtgassist you will find
Ghostly Prison as the first card (which is a white version of
Propaganda) and
Elephant Grass as the 7th card (which is a green version of
Propaganda).
Edit: Of course all rules have exceptions. These are just the basic for getting my deck started.
Edit2: I have trouble building green decks because green has so much options for everything.