Execution is the act of actually entering or exiting a basis trade on an exchange. You can trade this way by using different exchanges, but it is safer to use the same exchange when trading in this market neutral manner.
Its best to enter both the perp and the spot trade at the same time, and there are two ways that you can do this.
It can be tempting to try to ‘time the market’ when entering into a basis trade, but this can be quite risky. Timing the market on a basis trade is when you choose to short the perp and then wait to buy spot lower or vice versa. This is dangerous due to the fact that you could short the perp and then if price moves up, you are trapped in a short position. I’ve made trading mistakes trying to do this, and have found that it really isn’t worth it to try to chase a slightly better overall basis rate.