Will be using MadHQ's Gun Turret as example: http://zombapro.ppmsite.com/shp/shp/gagun.zip

Of course, anything and everything is up to you (you can copy-paste everything from Sentry Gun or Pill Box, even the Yuri Gattling Gun), except for these:
Code:
[GAGUN]
TurretAnim=GAGUNTUR
TurretAnimIsVoxel=no ;if your turret is a Voxel like the Sentry Gun, Flak Cannon, Patriot Missile, etc. make this yes, else keep this no
TurretAnim=GAGUNTUR
TurretAnimIsVoxel=no ;if your turret is a Voxel like the Sentry Gun, Flak Cannon, Patriot Missile, etc. make this yes, else keep this no
I hope you don't need to be told by this point that the GAGUN needs to be in the [BuildingTypes] list before it can be used by anyone. I'm mentioning it here just in-case you overlooked it; it happens.
What you might not expect is that GAGUNTUR needs to be listed under [Animations] if you want the game to render it.
Of course, same goes for the thing's Buildup= if you want to be able to sell it.
That was the rules part, here comes art.
The GAGUN's art settings can be whatever you want them to be. Just remember it's spelled Buildup=, not BuildUp=. You don't need to do anything special here.
You can copy-paste this from TS' Component Tower's Gattling upgrade.
Code:
[GAGUNTUR] ;copied from TS art.ini GACTWR_B
LoopStart=0
LoopEnd=32
LoopCount=-1
Rate=0
Surface=yes
NewTheater=yes
LoopStart=0
LoopEnd=32
LoopCount=-1
Rate=0
Surface=yes
NewTheater=yes
And you're done.
If Turret doesn't render properly try TurretAnimZAdjust=-20 in rules.
If that doesn't help go deeper into the negative, like -30. Or be extra lazy and just use -100 and be done with it.
Note: if no theater specific art is found, the SHP files where the second letter is a G is picked for use by the game.
In MadHQ's turret's example that'd be GGGUN.shp for the building, GGGUNTUR.shp for the turret. If you have NewTheater=no and use the same naming-scheme as the game (GAGUNTUR for MadHQ's gun turret) the game will only use GAGUNTUR.shp for the turret.
If NewTheater=no on the building itself (GAGUN here) then the main building is only rendered with GAGUN.shp for the building.
Since NewTheater default to no, you might want to ditch Westwood's naming-scheme and use GGGUN and GGGUNTUR for the IDs if you're that worried about drive space and don't want to use the snow-themed buildings (to save more space!).