Good ideas, knowing where to shop, and a good eye aren't enough to guarantee you a successful interior design project. Knowledge is the key, and your knowledge has to extend beyond the "stuff" that goes into an interior. You have to know what you want, know how to get what you want, know how to use what you get, and know how what you're getting is going to work for you and your space. To put that into an easier-to-digest sentence, knowing what you want is only the first step. Interior design is a process; it is not a concept that is put together at the snap of one's fingers. At the same time though, it is not a process that one should have to agonize painfully over for months and analyze to the point that it is no longer a pleasure.
Generally speaking, all design choices are made for either, or both, of these reasons: purpose/function/use, or aesthetics/appearance/look. The more your project leans toward décor, rather than design, the less important the purpose, function and use become. And the reverse is true as well. Shop interior design is the creation and planning of an interior space or room, while décor is the embellishing or ornamentation of that space. Décor can be applied to any designed space, but without the designed space you have nothing to decorate.
Prioritizing is the most crucial step of planning a project. Not only do you have to prioritize in terms of "needs" and "wants", sequence or scheduling, and budget, but also in terms of what you expect of the project's end result. The use of environment-friendly materials might be very important to you. Or you might insist that all materials and items are easy to clean. You might know that you are planning to move within three years so durability, or a timeless look, are not necessarily big issues. You might want to make sure that your design encourages family togetherness. If you have a problem with your shoulder it could be quite practical for you not to have anything located out of your comfortable range of motion. The possible "personal priorities" for a design project are endless, and you should invest a fair amount of time creating and reviewing this list.
At this point, you should have lists. Aside from your "personal priorities" list, you should have a list of "needs", "wants", "likes", and "don't likes." It is now time to make sure that what you are intending to do does not contradict with what you have established in your lists. Presumably, you are working with floor plans or other drawings; review the drawings as you go through your lists. What has been left out from your plans? Have you included things that are not important to you or that you really do not require?
Once you have made sure that your plans satisfy your needs and priorities, it is time to move on to the choices that will develop the décor of your design. Consider materials, looks, colours, textures, patterns, and such. They have to make sense with your design - it is the design that sets the stage for the décor. If you have a minimalist design, it would make no sense to fill it with busy prints, heavily textured items, and lots of stuff. When planning your design, it is important to be true to your preferences, because in planning the décor you must be true to your design to a great extent. An exceptionally simplified way of looking at this is that your design includes the items that are fixed in the space: the walls, floors, ceilings, cabinetry, built-in furnishings, doors, and so on. The removable items and the finishes applied to the fixed-in-place things are your décor. It is often said that "a good design speaks for itself," meaning that a well-designed space requires very little decorating work to be done for it to look great.
Part of putting the project together is knowing where to shop, and how to shop. Honestly, there is rarely a right or wrong place to buy something; evaluate quality and price to decide if you are in the right place looking at the right thing. Knowing how to shop is a little more complex sometimes. It is so easy to become frustrated at walking out of a fifth store in one day empty-handed. A good shopper will not purchase something that doesn't quite meet their expectations of it just because they are frustrated or tired. But you also have to know to be honest with yourself if it happens that you reach a point of indecision. There is such a thing as over-shopping when you are not quite sure of what it is you are looking for. When you head out to go shopping, keep as detailed a shopping list with you as possible. My own shopping lists often have things like "incandescent semi-flush-mount light fixture, maximum 15 inches high, stainless steel or brushed nickel finish, modern/transitional, textured shade - glass or other material." If I were to have trouble finding something that matches that description, any experienced salesperson would be able to direct me to the few choices that would suit my needs. Also keep a small tape measurer with you, as well as samples of colours or materials being used in your project - it really is amazing just how many shades of each colour exist, and the most difficult tend to be the off-whites and beiges.
The most complicated side of knowing how to shop is when it comes to the custom-made items. Think about every angle and aspect of custom-made pieces before you go to place an order: top, bottom, sides, inside, outside, upside-down if you must. Down or foam stuffing? Metal or plastic zipper? Three drawers or four? Left or right opening? One- or three-hole faucet? Polished or honed? Custom-made items do not have the same rigid pricing as shelf items in a department store, and the more information you are able to provide right away, the higher your chances of being able to get a better price. If the sales staff has to guide you through the options for finishing choices, materials, or other details, your price might be a bit higher because of the extra time involved.
It is not uncommon for suppliers of custom items to charge a fee for design work, and by helping inform you of certain choices they are in fact doing a part of the work they would be doing if they were handling the design of the item. Advice and information won't necessarily show up on your bill as a "design fee", but you will pay for it by not being able to talk the price as low as you probably would have otherwise. It is not advisable to have one person designing your cabinetry, another designing your stair railings, a third designing your window treatments, and so on. It is absolutely impossible that each person involved will have the same vision of your space, and the final outcome will suffer.
No matter your project type or size, planning ahead is imperative. Your whole concept can come about from a place you have seen, either in person or in a magazine, or based on what you know you like and dislike. But if you cannot organize your ideas in such a way that they act like a neatly laid path to follow throughout your design and décor project, there is no telling if you will actually end up with what you had envisioned for your space.