on a windows machine you will find the Jar file with all the assets in in the below location, the quickest way is to use the run command ( but if you are using windows 10 you can just type it in after pressing the windows button..

%appdata%

and then pressing enter.
when the new window opens, look for '.minecraft' (DOTminecraft)

.minecraft > Versions > [ VERSION YOU WANT TO CHANGE TEXTURES OF ] > [​VERSION].jar


to open these files you may need an unzip program, some use winrar but i use IZarc.
once you have the file open..

assets > minecraft

drag and drop the textures folder out of the jar file into your desired location and have a little look in there.
your nether portal texture will be in...

textures > block > nether_portal.png

if you wish to mess with the frame count or speed then you WILL need

nether_portal.png.mcmeta

that will be right next to it
-------------------------------------



Creating a texture pack (basics)



Lets start with the folder structure of the pack for just the portal texture



First find a nice place for your project folder and name it whatever you like, for now i will just call it 'MyPortal''



Now we need to add a folder in 'MyPortal' called 'assets' [[ no capital letters! ]]

in 'assets' you should create 'minecraft' [[ no capital letters! ]]

in 'minecraft' you should create 'textures' [[ no capital letters! ]]

and finally in 'textures' you create 'block' [[ no capital letters! ]]




Your folder structure should now be set up like below;

MyPortal > assets > minecraft > textures > block

The ‘block’ folder is where your ‘nether_portal.png’ and ‘’nether_portal.png.mcmeta’ will go.

Once the file(s) have been placed in that folder you can head back up to the ‘MyPortal’ folder.

You now need two files

The pack.png file (the small image that is next to packs in the resource packs screen) and..
The pack.mcmeta (which lets you add a description in the resource packs screen as well as let minecraft know which version the pack is for and upwards of.)

If you were intending to make it for the latest version of minecraft (currently 1.15.1 [ at the day of this post ]) you could open up notepad and copy and post the generic code below into it

Title of Expandable Spoiler

{


"pack": {


"pack_format": 5,


"description": "My Portal Texture!"


}


}

When saving the file make sure to click on ‘save as type’ and change it to ‘all files’

Now you should be able to safely name the file ‘pack.mcmeta’ [[ no capitals !! ]]
Save it in the MyPortal folder and check the folder to make sure that it IS correctly named.
(( if it is saved as ‘pack.mcmeta.txt’ then just edit the name and take off ‘.txt’ ))

(( if it is saved as just ‘pack’ then you will need to go into your ‘file and folder’ settings to show extensions so you can infact check that it is saved as the correct file ))

Now we move onto your ‘pack.png’...

Create (or if you dont have a good image editor you can just google) for an image that is square and divisible by 16 (16x16 or 32x32 or 512x512 etc)

Save this file in the ‘MyPortal’ folder as ‘pack.png [[ no capitals !! ]]

__________________________

You now have yourself a pack of which you can use in minecraft! (hopefully!! :D )

Now we can place the pack in the right folder in one of two ways

%appdata% > .minecraft > resourcepacks

Or

Load up minecraft > click ‘options’ > resource packs > ‘Open resource packs folder’

With the folder now open go ahead and COPY your ‘MyPortal’ folder into the resource pack folder.

Your pack should now be visible in the resource pack list in minecraft (if it isnt then you may need to come out of the resource pack window in minecraft and back in again or you just copied the ‘MyPortal’ Folder to the wrong place.

Activate your resource pack and create a new single player game (in creative mode) and give it a test!



Troubleshooting

I see purple and black squares instead of my texture!!!
You may have named the texture improperly OR the texture is in the wrong location/missing.

I see a texture but it shows all the frames at once!!!
The mcmeta is is missing/incorrect, also make sure it is in the same location as your texture file!!