Prepare the source artwork
Use filled or stroked vector paths, remove unnecessary detail, and decide which parts should be separate layers. Illustrator and Inkscape exports are useful starting points, but check that their groups make sense for the final symbol.
Fit the artwork to the symbol grid
A custom symbol needs consistent alignment and a sensible baseline. SymbolBuilder lands imported artwork on the symbol grid and provides alignment guides so the symbol can sit naturally beside system symbols.
Treat layers as meaning
Layer separation is what lets a symbol behave in hierarchical, palette, or multicolor rendering. Keep layers purposeful: use them for parts that genuinely need a different visual emphasis, not as an accidental by-product of the original SVG.
Export a real template
Export a standard SF Symbols template, then inspect it in the SF Symbols app or add it to an Xcode asset catalog. A live SwiftUI preview in SymbolBuilder is a useful early check, but the app where the symbol will live is the final test.