Is there any (official or not) guidebook to the creation of classes, both prestige and base, that does not simply state what you must pick for a class (BaB, saves, concept, profficiences), but actually helps in balancing the class, even though I find balance a really shady aspect of 3.5.
[RPG] Guidelines for creating homebrew classes (base/PrCs)
dnd-3.5ehomebrew
Related Solutions
As Flenyar quoted in his answer:
Each time he gains a new level, he chooses two classes, takes the best aspects of each and applies them to his characteristic.
This implies that if one class would gain a BAB at it's respective level, you do as well. Note that unlike normal multiclass characters, your stats do depend on what order you take the classes in.
Notice how Flenyar's progression (6 levels of Fighter/Rogue, then 4 levels of Wizard/Rogue) gives a BAB of +9, but the following progression gives you a full BAB of +10, even though the character is still a Fighter 6/Wizard 4//Rogue 10:
- Fighter 1 / Rogue 1 (+1 [Fighters gain +1 every level])
- Fighter 2 / Rogue 2 (+1 [Fighters gain +1 every level])
- Fighter 3 / Rogue 3 (+1 [Fighters gain +1 every level])
- Fighter 4 / Rogue 4 (+1 [Fighters gain +1 every level])
- Fighter 5 / Rogue 5 (+1 [Fighters gain +1 every level])
- Wizard 1 / Rogue 6 (+1 [Rogues gain +1 at 6th level])
- Wizard 2 / Rogue 7 (+1 [Both Wizards and Rogues BAB improves])
- Wizard 3 / Rogue 8 (+1 [Rogues gain +1 at 8th level])
- Fighter 6 / Rogue 9 (+1 [Fighters gain +1 every level])
- Wizard 4 / Rogue 10 (+1 [Both Wizards and Rogues BAB improves])
Total BAB at level 10 is +10.
This means that even a Wizard//Sorcerer gestalt (or any other two low BAB classes) can have a perfect BAB if you take just one level of another perfect BAB class:
- Wizard 1 / Fighter 1 (+1 [Fighters gain +1 every level])
- Wizard 2 / Sorcerer 1 (+1 [Wizards gain +1 at 2nd level])
- Wizard 3 / Sorcerer 2 (+1 [Sorcerer gain +1 at 2nd level])
- Wizard 4 / Sorcerer 3 (+1 [Wizards gain +1 at 4th level])
- Wizard 5 / Sorcerer 4 (+1 [Sorcerer gain +1 at 4th level])
- Wizard 6 / Sorcerer 5 (+1 [Wizards gain +1 at 6th level])
- Wizard 7 / Sorcerer 6 (+1 [Sorcerer gain +1 at 6th level])
- Wizard 8 / Sorcerer 7 (+1 [Wizards gain +1 at 8th level])
- Wizard 9 / Sorcerer 8 (+1 [Sorcerer gain +1 at 8th level])
- Wizard 10 / Sorcerer 9 (+1 [Wizards gain +1 at 10th level])
Total BAB at level 10 for this Wizard 10//Fighter 1/Sorcerer 9 is +10 (perfect.) This cannot be determined without the order of the levels taken.
However...
...the DM may overrule this, based on the wording of this rule:
Base Attack Bonus: Choose the better progression from the two classes. (Emphasis mine.)
It is conceivable, then, that the Wizard 10//Fighter 1/Sorcerer 9 is a character with 1 level of perfect BAB progression (+1) and 9 levels of low BAB progression (+4), leaving him with only a BAB of +5.
I'm not sure of any official errata on the matter, but it makes a big difference in some cases. Ask your DM, or make sure your players know your ruling if you are the DM. Personally, I would stick with the latter "better progression" rule over the "better increase" rule, since it seems to be the intent of gestalt.
There is also the "Fractional Base Bonuses" house rule, presented on Unearthed Arcana p.73, which is designed to allow smooth leveling of gestalt multiclass characters without any of these exploitable loopholes.
Saves
Since you mentioned saves in your comment, I'll touch on it briefly:
Saves would indeed work the same was as BAB, but beware that they are even easier to inflate artificially if you use the first presented "better increase" rule. This is because at first level of every class with good saves, that save "increases" from +0 to +2. Therefore it's even easier to end up with ridiculously high saves through multiclassing gestalt, if you don't simple lump all progression levels together before calculating character stats.
As a side note, consider two classes which gain 1d6 sneak attack every other level. Staggering them as I suggest staggering BAB increases still cannot double your sneak attack damage, since the book explicitly states:
Class features that two classes share accrue at the rate of the faster class.
Sure you can, as long as you meet the prestige class’s prerequisites. The Commoner class does not have any special rules about its advancement, it’s just a particularly weak base class.
It’s actually the easiest way to enter the Survivor prestige class (Savage Species), which requires that one’s highest base save bonus be lower than one’s character level. In a similar case, the Adept is the easiest way to enter the Hexer prestige class (Masters of the Wild), since it requires lightning bolt as a divine spell, and neither clerics nor druids get it, nor is it found on any Domain (amazingly).
I would comment that a common way to handle games where you start as Commoners is to have you “trade in” Commoner levels for PC-class levels, to avoid you taking pretty serious hits to your abilities relative to your nominal ECL. This is a non-issue if your DM knows how to compensate, but it might be worth mentioning as an idea to your DM.
Best Answer
The 3.5 DMG lists Modifying Character Classes and Creating New classes on page 174 and 175 respectively (PDF Doc).
There is also the Class Construction Engine from SHADOWCRAFT STUDIOS.
You can find more about Prestige Classes from Designing Prestige Classes - RPG.Net, and Designing Prestige Classes Part II - RPG.Net or Creating Balanced Prestige Classes - Guy Fullerton.