How would i get other mods to work in the modding environment? Like if i wanted jei or journeymap for testing.

Just put them in the run/mods folder if you just need them for testing. If you actually want mod integration then you need to add them as libraries through your build.gradle file or put them in the libs folder and add them as a library in your IDE.

I am getting method not found errors

Post them

I had the same problem and found the solution here: https://www.reddit.com/r/feedthebeast/comments/4zp2ku/is_it_possible_to_add_mods_to_the_1102_forge_dev/  (by brute force googling)

 

Go to Run/"Run Configurations ...", select in tree left "Java Application"/<Name-of-your-ProjectFolder>_Client, go to "Arguments" Tab and remove in "VM arguments" "-DFORGE_FORCE_FRAME_RECALC=true" what should be the whole content of that textfield.

This stops FML/Gradle to use any mod in its deobfuscated version; what means methods names, variables etc. in the foreign mod jar file don't get renamed into a human readable version while loading.
With deobfuscation the mod become unable to load parts of itself by reference the name of classes/modules. If you want to add references to other mods (e.g. for dependencies) in your own mod you might need a more elaborated method.

I would love to see the forge documentation extended by a section that explains deobfuscation much better than I try to do in the lines above ...

Still not working. Getting this from extra utils 2

Spoiler

// Don't be sad, have a hug! ❤️

Time: 11/12/18 7:07 PM
Description: There was a severe problem during mod loading that has caused the game to fail

net.minecraftforge.fml.common.LoaderExceptionModCrash: Caught exception from Extra Utilities 2 (extrautils2)
Caused by: java.lang.NoSuchMethodError: net.minecraft.util.ResourceLocation.getNamespace()Ljava/lang/String;
    at com.rwtema.extrautils2.api.machine.Machine.<init>(Machine.java:73)
    at com.rwtema.extrautils2.machine.MachineInit$1.<init>(MachineInit.java:92)
    at com.rwtema.extrautils2.machine.MachineInit.init(MachineInit.java:89)
    at com.rwtema.extrautils2.backend.entries.XU2Entries$66.<init>(XU2Entries.java:1154)
    at com.rwtema.extrautils2.backend.entries.XU2Entries.<clinit>(XU2Entries.java:1149)
    at com.rwtema.extrautils2.ExtraUtils2.preInit(ExtraUtils2.java:128)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:629)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91)
    at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150)
    at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76)
    at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399)
    at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71)
    at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116)
    at com.google.common.eventbus.EventBus.post(EventBus.java:217)
    at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:218)
    at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:196)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91)
    at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150)
    at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76)
    at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399)
    at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71)
    at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116)
    at com.google.common.eventbus.EventBus.post(EventBus.java:217)
    at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:135)
    at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:627)
    at net.minecraftforge.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:245)
    at net.minecraft.client.Minecraft.init(Minecraft.java:513)
    at net.minecraft.client.Minecraft.run(Minecraft.java:421)
    at net.minecraft.client.main.Main.main(Main.java:118)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at net.minecraftforge.gradle.GradleStartCommon.launch(GradleStartCommon.java:97)
    at GradleStart.main(GradleStart.java:25)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- System Details --
Details:
    Minecraft Version: 1.12.2
    Operating System: Windows 10 (amd64) version 10.0
    Java Version: 1.8.0_181, Oracle Corporation
    Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
    Memory: 386584288 bytes (368 MB) / 1065353216 bytes (1016 MB) up to 3790077952 bytes (3614 MB)
    JVM Flags: 0 total; 
    IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
    FML: MCP 9.42 Powered by Forge 14.23.4.2705 6 mods loaded, 6 mods active
    States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored

    | State | ID          | Version      | Source                           | Signature |
    |:----- |:----------- |:------------ |:-------------------------------- |:--------- |
    | UCH   | minecraft   | 1.12.2       | minecraft.jar                    | None      |
    | UCH   | mcp         | 9.42         | minecraft.jar                    | None      |
    | UCH   | FML         | 8.0.99.99    | forgeSrc-1.12.2-14.23.4.2705.jar | None      |
    | UCH   | forge       | 14.23.4.2705 | forgeSrc-1.12.2-14.23.4.2705.jar | None      |
    | UCH   | eob         | 0.1          | bin                              | None      |
    | UCEE  | extrautils2 | 1.0          | extrautils2-1.12-1.9.8.jar       | None      |

    Loaded coremods (and transformers): 
    GL info: ' Vendor: 'Intel' Version: '4.5.0 - Build 22.20.16.4749' Renderer: 'Intel(R) HD Graphics 530'
[19:07:20] [main/INFO] [STDOUT]: [net.minecraft.init.Bootstrap:printToSYSOUT:629]: #@!@# Game crashed! Crash report saved to: #@!@# D:\Desktop\MineCraft Modding\Empire Of Blood\run\.\crash-reports\crash-2018-11-12_19.07.20-client.txt

 

Wrong mappings. Extra Utils is written for a more recent 1.12.x Mapping, and you've set up your dev environment with an older version. Take a look at their build.gradle file for the mappings they are using