When I’m designing spells, or anything for that matter, my first step is looking for precedents within the existing rules system, which gives a really useful framework for how the new rules should work fitting in with the system, both in terms of power level and syntax – specifically how the rules are written. For example, in terms of spells, when creating my design for the spell Seeds of Strength in my Ravnica spell compilation, the precedent that I used as a base was Goodberry, as the two have a similar effect: growing a small berry or seed that can be consumed for a boon. (This carries across to other designs too, such as my design for the Spellbinder magic weapon, which uses similar rules to an item with a similar effect, the ring of spell storing.)
Another main early step for me when designing based on MTG is deciding whether to focused on mirroring the card’s effect as accurately as possible, or mirroring the card’s converted mana cost, with the effect being at a correct power level to fit with that level of spell. For the effect, I choose the appropriate level compared to similar effects on other spells. For simpler spells, I design the spell’s level to be the same as the card’s converted mana cost, then scale the effect appropriately.
There’s also a really useful section on creating spells on pages 283-284 of the Dungeon Master’s guide, it’s pretty handy for calculating the amount of damage or healing that a spell can do for its level.
As for my thoughts on the Plane Shift articles,
I’d definitely say that James Wyatt is one of my main inspirations for game design.
It was the Zendikar one that gave me the idea for my original homebrew posts designing Zendikar artifacts as magic items for a campaign I was planning on running back at that point.