I think I did about five full, 99% traditional (no lion belt, sad to say) Lesser Key evocations (with the Mathers ed.) before I started to see phenomena. It's not just about the "smells and bells." You've got to have determination and the will to do it, but you also have to pay attention to all the quiet intuitive things taking place around you (and by "around" I mean not just in the ritual chamber but in your life, which is the ultimate ritual chamber). In other words, there are subjective and objective dimensions to this work. The objective dimension is everything you've been gathering, preparing, enchanting, and observing: the tools, the timing, the place, the invocations, etc. The subjective dimension is what happens inside you and around you in the unseen aetheric world.
Here is a short anecdote about difficulty at the beginning. I had a teacher and was in a very small ceremonial magical group in the GD tradition. We weren't against demonic evocation, per se, but we were encouraged to explore the elemental realms first as a way to create resonances with them. We did that through tattva and tarot projections, mostly. But sometimes we did group guided meditations. I got to the point where I had met and interacted with the Elemental Kings, getting physical-world verifications of those reactions and recording everything. I also received a set of elemental familiars, which were the reflections of my soul in each element, essentially the part of me that is fire, water, earth, and air.
It was good and I felt like I'd improved as a result of that work. But its deeper value was that it trained me to interact with spiritual beings via my psychic senses. Instead of me standing in a circle, waiting for hell to open up and vomit forth devils, I realized that the unseen world interpenetrates ours (actually "interpenetrates" is not the best term because that still posits a degree of separation—there is no actual separation, even if we find it convenient to talk about Kabalistic Worlds or the Otherworld(s) as places distinct from the physical plane). All it requires is a psychic shift from your day-to-day method of seeing-thinking-understanding-acting. But that's hard to do. The traditional grimoire preparations and practices are there to facilitate that shift implicitly. Deliberately working on your psychic senses through astral projection, tarot contemplation, and / or meditation also greatly helps. This is where many get discouraged and give up. It takes time and effort.
After the elemental work, I started to quietly perform demonic evocation on my own. I think I was about 22 years old at that point. Through a combination of natural psychism and openness (and, I think, as a result of the strong elemental work I'd done), I fell into a kind of "transvocation," where the demon and I would appear astrally in a kind of staging area or middle ground. It wasn't the aetheric double of my magical chamber. It wasn't the spirit's supposed place of origin. It was always a kind of null space, an empty featureless astral location that I could strongly identify through semi-physical sensations in my third eye. Sometimes, those sensations are called "the tickling of the ant" but, for me, it felt like my forehead was cracking open. That's how I would know I'd "arrived." And there would be the demon. We'd converse. And then he and I would go our separate ways.
I worked like this for a while until I decided that I had to perform evocations properly, more by the book. Bringing my psychic senses into that work was a trial-and-error process and I failed about five times with five different demons. Then I started to notice my dreams changing. I had worked the evocation ritual from the Lesser Key (including memorizing the invocations, the Curse of Chains, etc.) so intensively over the months that I started rehearsing and actually performing it in my dreams—sometimes with results, which disturbed me at first.
One day, the dream reality and my physical reality came together and I began to see spirits in my chamber (but not necessarily bounded within the edges of my black mirror) with my eyes open. Since then, I have performed more simplified evocation ritual praxes (Huson's from Mastering Witchcraft remains a favorite and, for angels, Fio Praeter Humanus' lesser-known method). I nearly always get a response, but I don't always get special effects.
Bottom line: this is not a quick and easy process. But if you stick with it and let it change your life (meaning, if you pay attention to the subjective transformation with as much intensity as you have devoted to the objective preparations), you will reach a point where it is not so difficult and where you get consistent results.
In addition to astral projection, meditation, and tarot card mental projection, I recommend Konstantinos' psychic exercises in his Summoning Spirits and everything in Clint Marsh's fantastic Mentalist's Handbook. I have reservations about Konstantinos' book, but his exercises have worked for a lot of people and I've enjoyed them, too. Marsh is more interested in helping you develop well-rounded psychism on all levels.
Just keep in mind that spiritual evocation, whether you're dealing with Renaissance planetary intelligences, demons, angels, spirits of the dead, or some other entity, is a valid practice. It takes effort to become competent at it. But it is nothing to fear. And learning it can be very beneficial.