can you really tell a difference in C20 from C40?
How much is enough, or too much? [crystal?]
Is it best to use a SMaSh method to get to know the different malts and hops?
Additionally, I'll read some threads where people will make a recipe posted by someone else and enter it into a competition. Some win awards, but is that really your award?
I suppose what I'm really getting at is, how do you all come up with your own recipes?
How much experience is enough to confidently start with an idea and make it happen?
Perhaps in some recipes the difference in C20 and C40 can be discerned, but it's usually subtle and not a game changer. Difference from C40 and C80 is more noticeable, but still not a game changer. Most importantly, taste them side by side when you're at your LHBS, grab a few C20 grains and chew chew chew, mix with saliva, suck the juices from the grain, and record that experience in your brain. Then do the same with C40. You'll taste the difference but remember that will be diluted in the final beer so the change will be subtle.
How much cara/crystal malts to use really depends on the recipe you're brewing. Dry beers would generally use less than sweet beers. Styles will also help dictate how much cara/crystal to use. I'd say a general rule of thumb of how much per average 5g batch is 0.5-2 lbs with the majority being in the 0.75-1 lb range.
SMaSH can definitely help you hone in on certain ingredients. That said, you will devote a full batch to that one beer. IMO, SMaSH is best suited to 1 gallon batches. I do not believe that you have to do a smash recipe to get a feel for various ingredients and malts. I believe that by varying the malts and hops that you use, keeping your recipes on the simple side, that you'll gain a familiarity of what characteristics were imparted by which ingredient.
The using other peoples recipe and calling it yours question has arisen quite a bit. There are several lengthy threads about it and it really comes down to how YOU feel about it. You cannot forget to take into account the "beer brewing process" of the recipe which has a HUGE impact on the final product, and will only take place in your house with your system.
When I want to create my own recipe, from scratch, I start at the BJCP style guide and read about the style of beer I'd like to create a recipe for. There is A LOT of information that can be discerned from the style descriptions, not to mention they generally provide a 'typical ingredients used' section to help you tailor your choices even more. I'll then usually read about the style on BYO which also helps with ideas. After researching the style I'd like to make, I get an idea in my head of a beer that fits within the style (thinking hoppiness, maltiness, dryness, toastiness, roastiness, color, etc). After that, I think about my experiences with certain ingredients and how they might fit in to my recipe. Then I'll start reading ingredient descriptions for those ingredients I'm not too familiar with (beersmith descriptions and northernbrewer descriptions mostly). I then create a new recipe in beersmith, adjust my equipments, my brewhouse efficiency, and style selector. Once armed with a knowledge of the beer style and ingredient characteristics, I'll start putting a recipe together doing the best that I can with what I've learned. I always start with the grainbill and then proceed to hop schedule. My yeast choices usuall come from my current house strains that I'm working with (at present, scottish, notty, antwerp, farmhouse). It may take me 2-5 days to flush it all out, and after several tweaks I'm really close to what I'll be making. The yeast will really help dial in the final beer by it's attenuation, flocculation, ester profile, and accentuation properties. My last step is to take what i consider my final recipe and compare to some other recipes of this style of beer - usually on HBT. I just like to see how well I interpreted the recipe from style descriptions and ingredient descriptions as compared to others. At this point, I might make some subtle adjustments (mash temp, small specialty grain additions/subtractions, etc) but not so much that it's completely different from what I invisioned. VIOLA! I've just made my own recipe. I have found that my recipes almost always come out very decent to very good. Best of all, I learn about the ingredients I used and keep tasting notes for adjustments the next time I brew this beer.
Experience is something that's gained through having the experience. Therefore, the more you do it the more experience you'll gain. Confidence is gained through having the experience and learning from it. So to answer your question of "how much experience is enough to confidently do it?":
The amount of experience you have RIGHT NOW is enough!