Hello! Welcome to the OXC community. New members are always welcome, but new modders even more so... Especially those who make new graphics.
So, you're asking how to make OXC mods. Obviously this is a bery broad question, so let me cover some basics.
1. PalettesOXC uses traditional X-Com palettes (names are just how I call them):
Basic: for things that appear in various places, in tactical and geoscape parts (for example item images, like weapons)
Battlescape: for tactical combat (for example unit sprites, terrain, hand-held weapon sprites)
Ufopaedia: for big Ufopaedia pictures (not items)
Intercept: for interception screen
Minimized: for minimized crafts (following a UFO)
Basebits: for base view (facilities, crafts in hangars)
Backgrounds: for backgrounds, monochromatic
X-Com and TFTD use a different set of palettes, so each has two versions.
I've attached all palettes (for Photoshop) I have; these are for X-Com, not TFTD.
2. WeaponsA weapon (or any item) needs the following graphics:
a) bigob (large pic) - using basic palette
b) floorob (small pic for items on the floor) - using battlescape palette
c) handob (8 small pics for items held in hand, 1 for each direction) - using battlescape palette
This is fairly straightforward, only the handob is kinda tricky to get right - best paint over something that already works, or use some sort of template.
3. UnitsA standard unit needs the following graphics:
a) sprite sheet - the entire battlescape view, with all the body parts in different positions and the dying anim - using battlescape palette. Note that there is a number of routines that define how a sheet is used; for example a human has a different routine than a snakeman, not just different graphics
b) bigob - the corpse pic seen in inventory - using basic or battlescape palette (doesn't matter, because normally you don't see corpses outside battle; but battlescape palette is bigger, thus better)
c) floorob - the corpse pic seen on the floor - using battlescape palette
d) (optional) inventory picture (the paperdoll in inventory)
e) (when applicable) Ufopaedia picture, for example an alien unit (possibly also autopsy) - using Ufopaedia palette.
4. I have the graphics, what now?a) Check out this reference:
https://www.ufopaedia.org/index.php/Ruleset_Reference_Nightly_(OpenXcom) - it is big and not always easily understandable, but it contains a list of flags etc. for coding the resources into the game
b) check out some mods to see how other people did it
c) ask questions here on the forums (or the IRC)
d) Don't panic, we've all been there and survived.
Hopefully it's a start.