How best to plan and design a custom LEGO creation? I'm curious how best to proceed with my own creations. I have several ideas that I want to take a stab at implementing but know I probably don't have the all bricks I need. I am not new to LEGO, but I am definitely new to building high quality custom creations and am a little unsure where to start.
This is going to be different for everyone, but there are several different options.

I personally prefer to get my LEGO bricks out and just start building, this usually doesn't turn out exactly how I want initially, but I will take the bits I like and try again; building multiple versions to find what I like and come up with a finished item.  For larger builds, I will break it down into specific areas or elements to focus on and then work to integrate the sections when I am happy with each.

A very useful set of tools are the various computer Lego CAD systems, I prefer LDraw, the official Lego tool is the Lego Digital Designer, and there are others.

I have used these to start from scratch to generate a finished design, to generate a rough idea before moving onto the bricks, or to organize existing elements together into a larger build.

Depending on your experience they can be very useful tools not just for planning, but to generate build directions, parts lists, or high quality renders of you creation.  They will come with a large number of Lego pieces and colors allowing you to experiment beyond the Lego you have.  They are a little cumbersome for certain advanced SNOT techniques but can generally do most things Lego would.