This is the shortest part. Legendary creatures should be Rare or Mythic. The ones like the main character or leaders of factions should be mythics. While others should be rare. If they're is a legendary so good it should be mythic instead of rare, do it as long as you have the room.
Making rares is easy. Once you've made all the legendary creatures in the respective places, now you just have to make cards with good abilities (Like Daxos) or just strong (like Terra Stomper). These cards could be good alone or just work great within the deck the faction their supposed to be in is good with. These are usually the most fun to make, and you finish with them quickly if you get good art. Remember to balance the rares between colors, types and factions.
Making mythics is also easy. Since you usually have very little spots, about 7-10 of them should be already taken up. 5-8 Legendaries and approximately 2 Planeswalkers. Most sets only have 2 Planeswalkers because every block has 5, so it goes (2,2,1: usually) Remember with the new 2 set blocks, this could change so don't take this as a fact. For each color (and combo), you should have a very powerful spell or creature that has a great ability (Soulfire Grandmaster) or is linked to the story (Rakdos Return). Once these are done (which usually happens immediately), you should base a lot of other commons and uncommons off of these events or creatures.
Cycles are easy. You should know what they are, but if not they are 5 cards, one of each color (or faction combo) they all have the same CMC and mana combo (2W, 2U, 2B etc..). (This isn't always true for stuff like Commands, but it is for stuff like the Megamorph dragons.) They all have the same power and toughness if creatures and similar effect. The Dragonlord's Command cycles from Dragons of Tarkir are all instants and had Choose two:
But that was the only similarity. You should have a lot of cycles in the set, but don't make them for every card. Remember leaders are part of a cycle, of well leaders.
Well goodbye. Thank you for reading and using this tutorial. I hope it helped.