What Raveen said about the legality. I based my training core on CC and have stayed away from everything phoenix with no problems.
On to actually working on cores:
I don't have the energy to do a detailed guide so I will touch on the basics here.
ICE is located on the
Downloads Page (link) found up at the top of the forums here, or by clicking on the link I just made.
Its in a .zip folder so you'll need to open it with WinZip or similar program. No install is necessary, just copy everything out of the .zip into a directory. It does not have to be in your Allegiance folder either.
Core files are .igc so thats what you're looking for. I prefer to copy core files out of my allegiance directory and work on the copies in a separate folder so I don't accidentally save or tamper with the cores used for playing. You could alternatively just leave everything in the allegiance/artwork folder and make sure to save the core you're making under a different name that isn't being used right now (like "guivre_core01").
As for using ICE, its all pretty straightforward (fighters are under ships, max speed is clearly labeled max speed, etc.) except for pres & defs and a couple odd quirks here and there. The best way to learn to use ICE is to get into it with a core and just start experimenting with what changes what.
Quick guide to Pres & defs
"Pre" is shortform for prerequisite. On an item of tech it is everything that item needs to be available.
"Def" is shortform for definition. Its usually found on factions and development items (devels, or researched tech). These are the pre slots that this item fills out.
In ICE all of the pres and defs are listed for items. On each pre slot, only one item in the pre must be available for the tech to be available. However all of the individual preqs must be met.
Here's an example for pres & defs based on CC_04. I suggest you open up ICE to follow along as it makes what I'm saying much clearer.
Go into Parts and down to PW Gat Gun 2. Click on it, then go to the "Tech Tree" tab on the top right. This is where you find the pres & defs for any item of tech. For PW Gat2 to be available, it requires the items from pre #s 118, 157, 230, and 356. Thus it requires 4 separate items for the tech to be available to a team ingame.
118 is the weapon tech itself. Its made available by any faction with devel off (meaning in DMs) or by having researched Gat2 under supremacy, or by picking up a PE or PW Gat 2 that was floating around. This is what makes floating tech work in game - picking up Gat2 fills out the requirement in the core to make 118 available.
157 just defines the different factions, with both devel on or off. A team on any one of these factions will have gat2 available if it meets the other pres as well.
230 is the techbase, since Gat2 is mid level tech and generally requires a techbase to be available. If you look at the pre itself you'll see all the factions listed with devel off (so gat2 is available when development is turned off in settings) and all of the different supremacy stations. Remember that only one item in each pre has to be met to enable it, so just having a supremacy station will enable 230. As a side note, notice that Belters is also listed in 230 with development
on, meaning a Belters team doesn't have to have a supremacy station for gat2 to be available.
356 just defines the factions again, except TF, meaning that PW gat2 is not available to TF teams. This is because TF gets the PE version of Gat 2 instead.
Also note that in each of the four preqs that the factions are listed with devel off. This is what makes gat2 available to teams in DMs or any other game with development off in the settings. If even one of the preqs didn't have the faction(s) with devel off then they wouldn't get that tech in a DM (or other non dev game).
One more note: You'll notice that 118 is also in the "def" list. This is because we're looking at the actual PW gat 2. By defining 118, any team that picks the gun up and docks it at a base will fill out #118 for their team, and if they have a sup (or are belts) then the weapon is made available. In simpler terms, this is the mechanic that allows floating tech ingame to be picked up and used.
Hope this helps. If you have specific questions after playing around with ICE I'll try and answer those.