Pool based list design has become quite popular with Age of Sigmar, especially in the UK with Clash and SCGT being used at many tournaments.
The basic idea is that your total army is composed of units which are assigned a pool value and you then deploy a portion of your army as normal in the game until you reach the decided amount. With SCGT for example you have 150 total army pool points but only a maximum of 100 is to be deployed for the battle.
There are some unique considerations with designing an army list in this way compared to a more traditional approach:
Deployed force - Although you have a larger pool selection to select, you must remember that you only deploy a portion of your total army. You must design your list with the deployed amount I mind.
Battleplans - Most tournaments or even individual games with Age of Sigmar have unique deployment, objectives and rules. Each Battleplan you may play must be considered when designing your list.
Synergy - Although you could just pick units you like, it's best to have a wide selection of warscrolls in your total army so you can react to your opponent efficiently in deployment. Additionally, you don't want more of one particular Warscroll than what you're likely to deploy in one game. Why have 60 Freeguild guard if you are only expecting to ever deploy 30?!
Awards - Not gaming related but still important. You don't have to take the full amount of pools, just the amount required for the deployed force. If you have to fill your full army up with random, less well painted units you may miss out on painting points or a chance of best painted awards.
Sportsmanship - Also not really gaming related, but it's a good idea to have the choice of taking a softer or harder list depending on what your opponent deploys. If they have a fluffy and not so competitive army and you only have the hardest, toughest face smashing list you could design then you'll likely lose out on some sportsmanship points.
To summarise with some practical tips:
• Build a deployed force list for each Battleplan your are going to play then take any units which are the same across all the lists and add them to your full army. Now add remaining units from each list as best you can to optimise for each Battleplan.
• Consider the summoning rules for your games and amend your list if your require.
• Refine your list enduring you don't have redundant units that you wouldn't use.
• Consider changing your list to reflect the style of play you want and any sports or painting points you may be able to claim.
By no means a perfect way of designing a list for Age of Sigmar but hopefully this will give you something to think about when designing your lists for future pool based games.