The basic SaS build will vary some depending on map size and starting money (if more than $500)

If you are planning an early game SaS attack, or your base is located in the front, you might spend more on regiments and less on the HQ's (which bolster your economy). If you have plans on really getting your economy going, or your base is located in the back, you might spend more on HQ's and less on your armed forces. Of course this is an example of a multiplayer build (3v3, 4v4), if you attempt to eco this much in a 1v1 you would be quickly rolled...

There are two different types of formations, formation on the regiment level (Assault, Aggressive, and Column), and formations on the division level, which are a formation of a group of regiments.

Groups of regiments can also be put in the Assault, Aggressive, and Column formation by activating group behavior, but this is not recommended for combat, only for reorganizing your division if it becomes too spread out. That means most of the time your division should have custom formations with group behavior set to off. This also allows you to set the formation for each regiment in your division without affecting the group formation.

You will want to use different group formations for different game situations, some basic examples are seen on the left and right. The determining factors are usually what type of enemy regiments you are facing, and how many special op bombs your opponents have. Hotkeys are essential for effectively managing your divisions, so once you have selected and organized a group, save it by holding down [Ctrl] and pressing any of the keyboard number keys; or simply right click an spot on the group control bar.
 
Of course there are many other factors that influence a successful infantry campaign, see conclusion for more tips and links.

The SaS regiment is one of the most powerful infantry regiments, but is also expensive to produce. This means in certain matchups you will be outnumbered, for instance versus Russian snipers which can be deployed for as cheap as $60ea (Rokossovski). Find ways to buy time to produce more regiments, and entrench in key defensive positions if necessary.
 
If you happen to catch your opponents by surprise a small group of SaS can create havoc versus an unprepared enemy. If all they have is recon they'll be in a world of hurt... A single SaS regiment can be used to capture cities on the edge of the map which are usually secured only by recon.
 
It's up to you as commander to put your forces in an advantageous position with an understanding of base resistances, terrain modifiers, formation, upgrades, special ops, military tactics, and more.