sorry I haven't gotten back to your PM. I'll just answer here.
So, first it would be good to decide what kind of feeling you want in the room. Warm, cozy, spacious, modern, etc. This will play a lot into your color selection and the style in which you paint. If all of that sounds foreign then just skip it and jump ahead (and don't worry about scale and composition too much). Keep your design simple. If you plan to throw in little sorts of things and details and it's your first time muraling let those go until your base is down. Often times it's easier to come up with finishing touching once your base is on the wall and finished. As many artists will tell you, your direction will probably change along the way as you work on your project. Your concept at the beginning may very well be nothing like you concept once you've finished. This is how my project typically go anyway.
As for supplies. If you're planning anything large scale, thick, or dark colored I'd just go ahead and purchase wall paint for those larger items. For example, in ds room (the one you viewed in the other thread you pulled up) I used the same grade wall paint for the tree branches as I did for the walls. I find that it helps to use the same brand as often times different brands will have a different content makeup and make not coat well on top of one another. Don't used oil based paints as they will be very difficult to paint over in the future. Get your solid color (background color) on your wall and let it dry. Once that is done you can do one of three things. You can lightly trace with a pencil your design on the wall. Keep in mind that it is sometimes hard to wash pencil marks off completely BUT if need be you can paint over them with your origional color once finished with your mural. You can do as someone else suggested and use a projecter but for me that would be entirely too much work for me to even bother. That may be the best idea if you're not confident in your drawing abilities however. Lastly (this is what I prefer as I hate any extra steps) you can freehand your mural right on with paint and hope it turns out ok. If you have a wet washcloth handy you can wipe clean any fresh booboos.
If you want to do any shading or use more colors than you want to afford on wall paint you can head to the craft store for cheap acrylic paints. You can pick up 4 inch tall plastic containers (indivually sold in zillions of colors) for under $1 in some cases. Typically under $2 at least. You can even find them at places like Wal Mart in the craft section. They may be labeled as "craft paint" instead of "fine art paint". Basically it means it's lower quality but, when it comes to wall murals cost wins in my case. The paint is fairly thick and often times doesn't need much coating. It will thicken up as it sits outside the container (while you paint) so just add a bit of water to it as it becomes more difficult to paint evenly with it. Grab a set of paint brushes (again, go cheap) with various sizes. Makes sure there are very large brushes as well as very small. You'll probably end up using 2 or 3 different sizes. Clean them often as paint buildup as you're painting will begin to effect the smoothness of your product. You don't have to get it done all in one day. I'd suggest doing one wall at a time OR one layer (as in trees one day, branches the next day) at a time. In ds's room I did all of the tree trunks on each wall first, then added the branches once I had the trunks looking balanced. If you've mixed colors to make a particular color, make sure everything of that color can be done in one mixing so you don't have to try to recreate that color later. You can mix a large amount and seal it up in a small tupperware container for awhile if thats easier for you. I imagine it will be useable for several days if sealed well. You might just have to mix in some water to unclump it.
As for the leaves in the mural I did: I used fabric scraps. I used all different sorts of green scraps from my house and my mothers house. My mom made ds's crib set and matching quilt so I was able to snag those fabrics and use them for leaves as well. I just folded the fabric (single color) as many times as my scissors would cut and cut out simple leaf shapes. I'd do a different size for each different fabric so there was variation. To adhear them to the wall I used a photo grade double sided tape applied to the wall. The photo grade tape doesn't leave sticky film on the wall but is sticky enough to stay put. Then I used a fabric grade, fast drying, craft glue on top of the double sided tape to ensure that the fabric stay put. The tape alone wouldn'd hold the fabric well enough over time. Keep in mind that if you do this you'll have to keep the leaves high enough that baby cannot grab them as they will come down with little force. Cats also like them I've discovered.

: I've seen wall paper cutouts used for this sort of thing as well. They would definatly stay better and would come in lots of colors and patterns but, that might be expensive and they would be more permenate. I think I read somewhere that you can buy them precut in several different typical shapes like leaves. Maybe an online search would find some kits.
Just this week I did a mural in dd's room. She has one wall that is painted a solid, dark, cranberry color (the other 3 walls are a light lavender) and on that wall to one side I painted 2 large scale dandilion seed head with stem in solid white. I added a few flying indivual seeds across the wall as if they had blown off of the seed heads in the wind and were scattering. I didn't want to get too fancy and throw off the mood of her room so I kept everything solid white. Sometimes doing an entire floral mural in a solid color can make a more fitting statement. You might want to consider that as it also cuts down on cost and difficulty.
Ok, I think that about covers it! Let me know if you have any more questions or if I made no sense.