You will say no far more often than you say yes. It is the most frequent act in this entire process and almost nobody thinks about how to do it.
Say it soon
Within two or three days. The other family is waiting, hoping, and probably declining other conversations while they wait. Every day you delay costs them something.
A slow no is not a gentler no. It is a more expensive one.
Say it plainly
"We have decided not to take this forward" is complete. It needs no elaboration.
What causes damage is the reason people invent to soften it — the horoscopes, the timing, a vague mismatch. Manufactured reasons are usually seen through, and they invite an argument about the reason rather than acceptance of the answer.
Do not offer feedback unless asked
Telling someone why they were not chosen feels honest and is almost always unkind. They did not ask, they cannot act on it, and they will remember it for years.
If they do ask directly, be brief and be about fit rather than about them. "I didn't feel we were right for each other" is true and survivable. Anything more specific is not.
Do not disappear
Going quiet is the worst of the options and the most common. The other side is left checking their phone, constructing explanations, and eventually being told by someone else.
Two sentences prevent all of that.
If you are receiving one
It is not a verdict on you. Most nos in this process are about circumstance, timing, another conversation that was further along, or something in the other family entirely.
The people who come through this well are the ones who treat each no as information rather than judgement — and who notice that the process only has to work once.
You are not being scored. You are looking for one person, and everyone else is simply not that person.
One last thing
Be decent about it. Communities are small, the same names come round again, and the way a family handles a refusal is discussed far more than the refusal itself.