This year, I did it in two parts: sales last Friday and the buys today. This was because I now keep the majority of the cash portion of my portfolio invested in a no-load short-term bond fund.
After selling the gainers, I then calculated how much more money I would need to complete the buys, and then placed a sell order for the mutual fund on Friday. Today, the mutual fund sale proceeds were in my account, and I used them to complete the buys.