Do you mean what runes and masteries you should take or are you looking for a process to decide what your pages should be?
If it's the later, well I mostly just make a new rune/mastery page whenever I feel like I want one for a given champion in a given role. I don't have more then 20 things I'm ever playing regularly so this works fine.
If you're looking for specific advice for what to put in your pages there are lots and lots of guides out there. Really it's going to come down to a matter of personal play-style. Some of my favorites are:
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Runes
My default rune page
Marks - 9 Hybrid Penetration
Seals - 9 Armor/Scaling Armor
Glyphs - 9 Scaling Magic Resist
Quints - 3 Movespeed
On mages the physical pen helps with csing and harass early on and there are very few champions who deal ONLY physical damage. Hybrid Pen may rarely be the best choice but it’s almost never a wrong choice. Armor and MR help everyone. As a tank or fighter? Extra resists are never bad. Squishy? You’re spending all you gold on offense so a little defense from rune can really make a difference. Lastly you want to know which champions benefit from extra moves peed? ALL OF THEM.
Default with 5% cdr
This is the same as my default page but I swap out 3 Glyphs for scaling cooldown reduction. The reasoning is the same as my default page applies but those 3 Glyphs give 5% cdr in late game to match the 5% cdr from masteries so I can hit the cap without going over.
All Health
Marks - 9 Armor
Seals - 9 Scaling Health
Glyphs - 9 Scaling Magic Resist
Quints - 3 Percent Health
The flat armor Marks are just about as good as flat armor Seals for early game tanking. The scaling MR is also good for tanking. Scaling Health and Percent Health synergies and work well for health stackers like Volibear, Naut, or Sej.
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Masteries
I use too many variations to really say much here. Just remember you don’t need to go 21 and 9. Sometimes it’s better to go 15 and 15. One I will mention is
Tank tank tank
0 / 30 / 0
This page is pure defense. I generally couple it with my Health rune page above when playing tnaky supports to be the most unkillable tank in the field.