Finger or stylus

Both work. The difference is what the screen can feel.

A stylus

An Apple Pencil or an active pen reports pressure and tilt, so your line thickens where you pressed and thins where you lifted. That variation is a real part of your hand, and it's the main reason a pen-written font reads closer to your writing on paper.

A finger

A fingertip reports position and nothing else. Inkling doesn't invent pressure it can't measure — inventing it would be inventing your hand. Instead it sets an even line weight suited to finger writing, so the result is honest: a clean, consistent line rather than fake pen dynamics.

Finger-written fonts are good. They're a little more even than pen-written ones, and on a phone you'll write larger than you would on paper, but the letterforms are yours.

Pick one and stay with it

This is the part worth knowing: keep to one instrument within a font. Inkling notices which you used and keeps the weight consistent, but a font written half with a pen and half with a finger can read unevenly — some letters carrying pressure variation and others not.

If you start on your phone with a finger and later get a tablet and pen, you're better off starting a fresh font than mixing the two.

Palm on the screen

Rest your hand normally. Inkling ignores your palm and takes the pen. If stray marks are appearing where your hand rests, see The canvas won't take my writing.