We upholsterers are humans like everyone else. We each have out own weaknesses and failings. Even in the upholstery trade each of us has our own areas or expertise and areas of weakness or lack of skill. When clients come into our shop, or when we go into their homes or places of business, they want to know that we are competent professionals who can do their jobs.
In the midst of this we have to be real and honest, both with ourselves and with our clients. If a job is way out of our area of skill or knowledge, we need to know when to say "no" and to hold firmly to it.
In a large part when we interact with a client we are acting in a play. In other words, we are "acting as though we are knowledgeable and competant" even when we are quivering and fearful inside. For example, when we have to give a price or an answer about something that we know nothing about, we might be inwardly quivering and saying to ourselves "What do I do about this? I don't have a clue!" <grumble, grumble, grumble> Even so, it does not help our relationship with the client to let the client see our uneasiness and fear. So, no matter what is going on inside we "act as though" everything is fine. Now, in the midst of all of this inward turmoil, and our playacting, we balance that with being real and authentic with the client. We only tell them the truth. (At the very least, we at least don't tell them any lies). We be as real as we can be.
Then, you might ask, "When the client asks me questions and I don't know what to say, what do I do. First, Tell NO lies. You can start with clearly defining their questions. Write them down. Then tell them, "I'll find that out for you." all the while acting like a professional.
In any trade or career there will be things that the people don't know and have to find out. That is a normal part of living. So don't stress out about not knowing or thinking that you will look foolish if you don't know.
Appearing competent is not about knowing all the answers. No one does that. Rather it's about being real and honest while keeping calm and asking the right questions so that you can find out the answers.
What about mistakes? Does that mean that I'm not competent. Of course not. Everyone makes mistakes. Not one is born knowing everything. The differences is that an amatuer may try to hide or cover up his make hoping that the client won't notice it. In contrast a professional more quickly notices his mistakes and promptly corrects his/her mistakes. When a mistake is corrected, it's as though it never happened.
In order to "appear competent" one must consistently act in a competent responsible manner. At first it may be "an act", but with continued repetition it will become second nature and you be truly competant