Along with my coursework, I did three professional internships during my summers in NY and LA - at a boutique architecture firm, a large architecture firm, and the real estate department of a large corporation. By the time I graduated I had the confidence, skills, and resume to start as a Junior Designer in the Corporate Interiors department of a large architecture firm.
Then comes the boring part. I worked at 3 different companies from 22-31. I worked on all manner of projects. Working in the corporate sector is less creative than one would like but you learn everything A-Z about how the pieces come together in a project both from a planning and a construction standpoint. At 31 I had a small baby at home and it made me really sad to be away from him, working for others where I felt unappreciated and uninspired. Like, what am I doing this for? To be honest I had wanted to go off on my own starting at age 25, but I didn't have the confidence that I had enough skills to carry my own projects, or enough money saved up to pay my expenses while I was building a business. It was at that point in my life at 31 that A) I had the confidence in my skills and B) I was married to a supportive spouse who encouraged me to take the leap. He could carry us financially while I was building up my clientele and I could be home with the babe. If he didn't believe in me, it probably wouldn't have happened.
The first couple of years were slow but I also wasn't in a rush. I had a toddler and then a second baby so there was only so much I could handle anyway. I developed my voice and the clients who "got" my style found me. I did very little marketing besides a website and a Houzz profile. The more projects I completed and professionally photographed, the stronger my brand became, the more consistent my work load got, and about 2 years ago I felt like I had succeeded in the goals I had originally set for myself. Now I'm busy busy with awesome projects and cool clients. I only take on projects I feel excited about because again, I have two young kids at home and I want to be there for them. I only work with people who I like because I'm not desperate for work. I also have a lot of confidence in my talents and I have a viewpoint that people are lucky to work with me. Life is too short to work with people who stress me out and don't appreciate my services. I know I sound so obnoxious and privileged, but it is what it is. I know not as many people can be so lucky. I get that.
And that brings me to why I am not the best person to give career advice. Because I didn't decide I wanted to do this at 31 and started my own company with no previous experience. Because I studied, and interned, and worked boring corporate jobs for a decade before launching my own firm. That's not to say it can't be done. Lots of designers have no formal training. I just wouldn't want to take on the stress of that steep a learning curve. When someone is spending $20-$200k on a remodel, that's A LOT of responsibility. Yes, its super fun to pick out tile and light fixtures for them, but you need to know how the details come together and anticipate what can go wrong so things flow seamlessly. When things go south, and it was you who recommended a certain application or product, you are on the line for making it right. I hate that part of running my own design firm, but its worth it for me because I know I have the knowledge base and things will go smoothly. And if they don't I know how to problem solve. I'm actually pretty impressed with designers with zero training or past experience who decide to go for it. That's ballsy.
My advice would be: if you don't know how to draw a floor plan or elevation in AutoCAD, create a 3D rendering in SketchUp, make a design board in Photoshop, read a set of construction documents, know local building codes, and be super organized to track orders and specifications in Microsoft Office, then you probably shouldn't start your own design firm. Being able to pin pretty pictures and shop is probably not enough. Your clients will expect the former over the latter. My obvious advice would be to take some classes first to learn those basic interior design skills. Learn the software. Then get an assistant job or internship at a design firm to see a few project cycles in person. Interior design is super gratifying and it can be fun, for sure, but there are many stressful and draining aspects of it like any job. Its not all rainbows and unicorns and you should see it in person.
If you are in your 20s and are considering going back to school to get your graduate degree for interior design....I recommend you start with a job in the design industry before committing to being a designer. There are actually many support positions that give you insight into how it all works along with daily contact and collaboration with designers. Jobs like a sales rep for a product line (furniture, fabric, lighting), working as a sales consultant in a high end showroom, or working in the business or marketing departments of large design firms come to mind. If you did this for a year or two after graduating you would get a steady professional salary while you figure out if being a designer is for you. If it is, then you apply to design school and make that commitment without going in blindly. If its not, you haven't taken on more debt and you have built your resume in the meantime.
If you are in your 30s and beyond, you may not have the same time flexibility. You need to get something going stat to pay your bills. Its a pretty saturated field of competing interior designers, some with experience and training and some without. To establish that you are serious and passionate about interiors the first thing to do is start a blog/website. Write about the interiors that interest you and define your style. Of course, I highly recommend taking classes whether its a formal program or a boot camp type weekend seminar to teach you the basics. Offer to design your friends' spaces for free and take good photographs to build a portfolio. A Houzz profile is key so update it regularly and clients will find you. Start by asking pretty low fees just to get professional projects under your belt. As your projects get more complex and better looking, submit them to interiors blogs and magazines to get exposure.
Because I am risk averse, I would take on only decorating projects first before tackling other people's kitchens, bathrooms, and additions. If you really believe in your style but know you lack the skills to illustrate your concepts, you can always hire out drafters and renderers to create the drawings for you. I actually know a few designers who do that, but then their fees have to account for the outside work, and they have to be super confident in their creative input to justify the pricing of their output.