Most rooting failures come down to one mistake: cutting in the wrong place. For vining plants like pothos and philodendron, roots only form at nodes — the small bump or aerial-root nub where a leaf attaches to the stem. A cutting with no node on it, no matter how healthy the leaf looks, will not root.
The basic method: using clean, sharp scissors or snips, cut just below a node, keeping at least one leaf attached above it. Remove any leaves that would sit underwater or below the soil line — leaves left to rot in water foul it quickly and can kill the cutting before roots even form.
In water, change it every few days, more often if it starts to look cloudy, and keep the cutting somewhere with bright indirect light, not direct sun, which can cook a leaf with no root system yet to support it. Roots typically show within one to three weeks for easy rooters like pothos; more temperamental plants can take longer.
In soil, the same node-below-the-surface rule applies. A rooting hormone isn't necessary for easy plants, but it can improve success rates and speed for slower ones. Keeping the soil consistently barely moist — not wet — and the humidity a little higher than normal (a loose plastic bag over the pot works) helps more than any product.
The single biggest variable is patience: check for resistance by giving the cutting a very gentle tug after a couple of weeks rather than pulling it up to look.