Scaling out of positions: best practices?

Hey everyone, I've been paper trading $EURUSD and $GBPUSD mostly, trying to get a handle on risk management. I understand why you'd want to scale out of a winning trade, but I struggle with how to do it effectively without giving back too much profit or exiting too early. For those of you who scale out, do you have a set percentage at certain price levels, or is it more discretionary based on price action as it develops? Also, do you adjust your stop loss on the remaining position differently? Just trying to refine my process.