Learn the small decisions that make a dish work. Cook with what you have, get comfortable making substitutions, and spend on ingredients and tools with a purpose.
Understand the job before buying the tool
What problem are you trying to solve: a dull knife, uneven heat, or simply not enough prep space? Name the problem first. A useful purchase should solve something you actually encounter.
Use substitutions with a purpose
An ingredient can supply texture, fat, acidity, sweetness or structure. Think about which of those jobs you need to preserve. Baking and preservation can depend on precise ratios, so use a reliable tested recipe for those changes.
Make notes for next time
Record what you changed and what you would repeat. The most useful recipe collection is the one that reflects your kitchen, your equipment and the people you cook for.