Titan
Titan is fairly simple to build, and unless otherwise noted all parts for Titan I begin with the name Prometheus-I, Titan II parts begin with Prometheus-II, and Titan IV parts begin with Prometheus-IV. Titan III is a combination of Titan II and Titan IV parts. The Atlas (Bossart) 1.875 decoupler and fairing base are used frequently here.
Other than Titan I's 1.5m second stage and the "Sultan" booster these are all 1.875m parts.
There are three generations of engine pairs with switchable models, and five generations of large solid boosters. Auto struts or KJR are mandatory when using the solid boosters due to the weight.
Though the real Titan I never orbited anything, this version is quite OP and will put payloads in to orbit. Comes with a 1.5m fairing base or a "warhead" probe core with a materials bay experiment built in.
There are 3 variants of the Titan II. The famous Gemini Launch Vehicle, the ICBM, warhead not included but there is a special fairing base for it, and the Titan 23G which uses the Atlas (Bossart) 1.875m fairing for the optional Star 37 third stage and payload. Titan II is also a bit op, but the extra fuel weight works against it. For best results reduce the second stage fuel to 65%, and the first stage fuel to 90%. Also reduce the second stage thrust to 63% to extend the burn time so there is no coast before reaching orbit. Or if you prefer the TitanII_GLV folder in Extras contains a patch that will clean up Titan and Gemini until these parts are updated.
The Titan III core is Titan II tanks fitted with Titan IV engines, and topped with a Transtage (Prometheus-T) third stage. Variants will swap out the third stage for an Agena D or Centaur D-1, or use no third stage at all. A pair of 1205 boosters (Prometheus-III-5) are typical.
Outside of the Saturn family and possibly the proposed Atlas V Heavy, the Titan IV carries the heaviest payload to orbit in this mod. The Titan IV brings it's own set of larger core tanks, and a pair of 1207 boosters (Prometheus-III-7) for the Titan IVA, or a pair of SRBU's (Prometheus-IV-SRBU) for the Titan IVB. A Centaur T third stage is installed inside the large fairing. Titan IV launches with the boosters only, then air starts the core first stage 30 seconds before the boosters burn out.