It seems to me that your question is two fold. I may be wrong but you seem to be looking not only for a technical point of view but also from a role-play point of you.
The technical aspect is quite easy to answer:
You will need two items to produce your concoctions(extracts, mutagens and bombs):
Kit, Alchemy Crafting
Price 25 gp; Weight 5 lbs.
An alchemist with an alchemy crafting kit is assumed to have all the
material components needed for his extracts, mutagens, and bombs,
except for those components that have a specific cost. An alchemy
crafting kit provides no bonuses on Craft (alchemy) checks. (This item
was previously called an “alchemist's kit”, and was renamed to avoid
confusion with the set of adventuring gear called an “alchemist's
kit.”)
The second item is your
Formula Book
An alchemist may know any number of formulae. He stores his formulae
in a special tome called a formula book. He must refer to this book
whenever he prepares an extract but not when he consumes it. An
alchemist begins play with two 1st-level formulae of his choice, plus
a number of additional forumlae equal to his Intelligence modifier. At
each new alchemist level, he gains one new formula of any level that
he can create. An alchemist can also add formulae to his book just
like a wizard adds spells to his spellbook, using the same costs,
pages, and time requirements. An alchemist can study a wizard's
spellbook to learn any formula that is equivalent to a spell the
spellbook contains. A wizard, however, cannot learn spells from a
formula book. An alchemist does not need to decipher arcane writings
before copying them.
Extract's preparation
How to make an extract. Most of the text around extract can be reduced to the funny (or not) "spell in a bottle".
In many ways, they behave like spells in potion form
[...]
An extract, once created, remains potent for 1 day before becoming inert, so an
alchemist must re-prepare his extracts every day. Mixing an extract
takes 1 minute of work—most alchemists prepare many extracts at the
start of the day or just before going on an adventure, but it's not
uncommon for an alchemist to keep some (or even all) of his daily
extract slots open so that he can prepare extracts in the field as
needed.
Alchemist extract are like spell really but with a kick they don't take as long to prepare, unless you have more than 60 extracts and at this point well... breaking action economy should not be too hard.
The downside is that you need to redo it every day so no storing for long winter nights!
Mutagen's preparation
Now comes the preparation aspect on how to make Mutagens. As far as I can tell, the usual in D&d is to be quite abstract and left to the imagination of the player the details. Most of the time for spell caster you have to go into splat book and descriptions to get a "look and feel" and not only the "use a standard action".
Mutagen (Su)
At 1st level, an alchemist discovers how to create a mutagen that he
can imbibe in order to heighten his physical prowess at the cost of
his personality. It takes 1 hour to brew a dose of mutagen, and once
brewed, it remains potent until used. An alchemist can only maintain
one dose of mutagen at a time—if he brews a second dose, any existing
mutagen becomes inert. As with an extract or bomb, a mutagen that is
not in an alchemist's possession becomes inert until an alchemist
picks it up again.
Emphasize mine... A mutagen is brewed, so it is likely to be prepared like a potion or extract but being far more potent it takes a whole hour to make one dose and the alchemist aura can only "power up" one at a time.
From the Alchemy crafting Kit, we know that you have all you need is in it.
So you now have the physical elements and you know that it takes you an hour to brew it.
I hope this will be sufficient for you to feel comfortable using your mutagen.
Making Better Poisons
I recommend taking a gander at Arsenic and Old Lace, a poisoner's handbook written for 3.5 DnD. While the poisons rules were changed alot in the creation of Pathfinder, it will be a handy guild for thinking of things you can make poison out of, their effects, and a relative pricing schema. Whether you want to stun, sicken, nauseate, paralyze, render unconscious/sleeping, slow, exhaust, daze, confuse, blind, induce a disease, render mute, or just suppress the scent ability it will have a poison for you.
These make great jumping off points for discussions with your GM, if nothing else.
Making Poisons Better
The best class for being a master poison maker, is as far as I can tell Alchemist. There are quite a number of class features and alchemical discoveries that make you a great poisoner.
- Celestial Poison lets you poison undead and evil outsiders,
despite their inherent immunity to poison.
- Concentrate Poison lets you combine two doses of the same poison
to increase the DC by 2 and the duration by 50%.
- Malignant Poison lets you increase a poison's DC by 4, extend
it's duration by 2 frequency increments, and removes the onset
time entirely.
- Poison Conversion lets you change if a poison is a contact,
inhaled, ingested, or injury poison.
- Sticky Poison lets a poison you apply to a weapon stay on it for
your int mod number of strikes.
To round this out, you can also get poison bombs, which are loaded with cloud kill effect, and the Poison Touch Grand Discovery to have the spell Poison on hand when ever you need.
Best Answer
The short answer is, the rules suggest ignoring these kinds of “fitting” problems, and therefore do not provide much in the way of rules for solving them. They do touch on some niche cases that might prove useful as examples of how your DM might houserule a modification process, which I’ve detailed below. But ultimately both Pathfinder’s authors and myself personally recommend against such houserules:
Moreover, the Alchemist itself doesn’t suggest this should be a problem:
Magic clothing interacts with the tentacle in special ways just because it is magic. Seems to me that magic armors, much as they resize so the stockiest dwarf can wear the armor of the twiggiest elf, also can accomodate the tentacle.
So really, this is not supposed to be an issue, the rules don’t explain how to deal with making it an issue, and it seems an unnecessary nerf to the Alchemist class. If your DM is houseruling this, he also has to houserule how such modifications may be made and what they cost. The rules don’t cover it.
Full Plate
The rules only acknowledge “fitting” issues in the case of Full Plate; in all other cases one-size-fits-all-in-Size (that is, anything that fits a Medium humanoid-ish creature will fit any other Medium humanoid-ish creature). For Full Plate,
No DC is given, though “master” might imply “someone capable of making masterwork items” which is a DC 18. Alternatively, it would be odd if such modifications are more difficult than making the armor from scratch, so just use the original DC. The cost is exorbitant for anything but Full Plate, since Full Plate is far-and-away the most of expensive of the core armors. On the other hand, the values range from just over ⅛ to just over ½ the base cost of the armor, so maybe those might be useful guidelines.
Armor for Unusual Creatures
There are rules for nonhumanoid characters’ armor, but in this case they’re talking about things like horses, which need a great deal more armor in a completely different style, not just an extra slot for an arm. At any rate, here are those rules:
There are no rules for converting regular armor to unusual-creature armor. Considering the creatures that these rules are intended for, such modification is probably impossible.