When I sit down to build a list, it is typically something I try to get done as quickly as possible. There are certain parts of the hobby I enjoy. Those are playing the game and talking about it with my friends.
Then there are certain parts of it I do not enjoy. Generally, that means everything else. Specifically it includes setting up and taking down the game, painting, assembling figures, and list building.
So typically when I set out to build a list I have a general idea of what I want to concentrate on. For example, in the list for the game pictured (a 7th edition list) I had just completed painting the Dragon Ogres, the Marauder Horsemen on the gray horses, the chariot, and modifying a Brettonian into my Standard Bearer.
As a result, I wanted to include those elements. I always include a unit of Knights because I love magic and knights. By the time you put in a few knights, some dragon ogres, a chariot, the requisite wizard, and a chariot, you have eaten up close to 1000 points. I believe that one was probably a 1500 point game.
Since I found Marauders in 7th edition to be worse than completely useless since they seldom ever got to attack, it was an easy choice to make it an all-cavalry force.
At that point my list is built and I simply need to figure out how to use it on the field.
When I build a list, this is a pretty typical example. I seldom take into account who my opponent is. In fact, I generally go somewhat out of my way to avoid knowing which army my opponent will use since then it is a truer test of my abilities.
I do, however, put in a battle plan along with my list.
For example, in the above list, the Knights would go straight up the middle while the Marauder Horsemen would guard the flanks. The Chariot and Dragon Ogres would lag slightly behind, planning to either attack anyone who flanked the Knights or, more likely, when the knights swung towards one flank or the other, they would combo charge on the appropriate flank while either the Marauder horsemen or remaining unit would serve as a blocking unit.
Every list I build has similar elements. I find a unit or concept I want to build around, fill it up, then when I have fit in the things I enjoy using, I fill in the "required" slots.
Case in point; I really like my troops to be reliable. As a result, the Crown of Command has quickly become a staple on those (extremely) rare occasions I do not take the Warriors of Chaos item the Banner of the gods.
However, one army (the Dwarves) has a second version, the Master Rune of Kingship. Add the Master Rune of Gromril and the Rune of Resistance and your General will make whatever unit he joins Stubborn.
Add a Thane with the Crown of Command and a BSB and suddenly your battle line is full of T4, LD 9 or 10 re-rollable stubborn troops. The ultimate in reliability short of the crumble-prone undead.
Depending on what points level you are playing, you can still have more than sufficient shooting to soften up the enemy before they crash into your nigh-impenetrable rock of a line.
Of course, despite the improved charge ranges, the Dwarf army is still enough slower (25- 50% slower than virtually any other army) that you need a solid plan.
Set up an Organ Gun between Dwarf Warriors and Dwarf Longbeards with cannons and Thunderers added as appropriate and you have a rock-hard, deadly army. The list built itself. It simply requires a few tweaks for points.*
In short, to build a list, I do the following.
1) Find a concept to build the list around.
2) Build a battle plan around that concept.
3) Include the pieces to fulfill that concept.
4) Add required things such as core troops.
5) Fiddle with the points until they fit.
*Note it DOES leave the army susceptible to magic, but this is somewhat deliberate. I like to always build in a weakness the savvy opponent can exploit.