How do you make a custom sword?

Well, it depends, what do you mean by a custom sword?

If you mean what I think you mean, a sword made of a custom material, then it is quite simple!

STEP 1: Create a texture! Easy as that!

STEP 2: Create a new tool.

STEP 3: [BLANK] Sword. Just put in your texture and name your sword.

STEP 4: Specify that your tool is a sword, and tell MCreator which Creative Inventory slot it should appear in.

STEP 5: Set the Sword's stats.

STEP 6: (Procedures, if wanted. There are plenty of tutorials on here for that)

STEP 7: DONE! Your new sword is complete!

OPTIONAL 8th STEP: Set up recipes to make your sword, this is simple enough, as MCreator's recipe making tool is very easy to understand...

You could make it inside MCreator, but it would be ugly... As such, I recommend Paint.NET. It's free, easy to use and has many powerful tools. ADVICE : if you want help for texturing your sword, try using Minecraft's base model and build on it (color it well and add some new bits to it or make it thicker or thinner or whatever as long as it makes it original)

I'll join in and combine all the comments into one big one:

In MCreator you can make a new Sword by making a new "Tool" element. You can name it whatever you want, just make sure it has no spaces, as this will reference the mod's internal name in the code. Underscores can be used to escape spaces.
 

After giving the tool a name, it'll ask for a texture. There's more than one way to get textures:

1. You can make it yourself using any program such as Photoshop, Paint.Net, GIMP, etc. Just make sure to stick with a 16x16 style size. (16x16, 32x32, 64x64, etc.)
2. You can use MCreator's built in Tool Creator.
3. (Controversial) You can use websites such as this or take inspiration from other sources, such as other video games. **If you do this, give credit to the source material.** My mod uses textures from Terraria and Novaskin, and using the /credits command will show all credits to asset sources.

As for Render type, keep everything as it is, unless you have a 3D model for a tool.
 

The next page is pretty self explanatory:

• Name in GUI:
  • The name of the Item when you highlight it with your mouse or switch items in your Hotbar.
• Creative inventory tab:
  • This is optional, but this tells the mod where to put the item when you're in Creative mode. Since it's a sword, sticking it in the "CreativeTools.COMBAT" tab would make the most sense (unless you have your own custom tabs).
• Type:
  • This tells the mod what type of tool it is, whether it be a Sword, Axe, Pickaxe, Shovel or a Hoe. There's also two others called "Special" and "Multitool" but I haven't personally used those and tend to stick with what I know.
• Harvest Level:
  • AFAIK, this only contributes to the Axe, Pickaxe and Shovel. This can be left at 1 for Swords, however there's a weird limit on how high this value can be. This can be circumvented by editing it via locked code.
• Efficiency:
  • ​​​​​​​Another attribute that really only helps for Axes, Pickaxes and Shovels. Basically makes blocks break faster. Default is 12.
• Enchantability:
  • ​​​​​​​This determines how likely you are to get great Enchantments using an Enchantment Table. Higher numbers grant better Enchantments.
• Damage vs mob/animal:
  • ​​​​​​​This determines the amount of damage to deal to an Entity in half hearts. (I believe it's half hearts.) Default is 6, so 3 hearts.
• Number of uses:
  • ​​​​​​​Self-explanatory. Determines how many times the tool can be used, however there's a weird limit on how high this value can be (internal max is 4,000). This can be circumvented by editing it via locked code.

Finally, we come to procedures. At this point we don't have any use for procedures for the tool, so we can just hit Next and you're all done! Just wait for the mod to compile and you're set. Although you do need to make a crafting recipe for the tool to make it craftable, just keep it in mind.