Before we start, you should know that this guide assumes you know how to use the Payday Mod Tool and that you've already extracted the game's files. If not, check out the Official Payday 2 Modding Guide and familiarize yourself:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=231568439
Also note:

• I'll make a couple assumptions about things here and there. The things I state about these files might not be 100% correct.
  
• I wrote this guide because a couple people asked how to change the menu animations.
  
• I tried to keep this as straightforward as possible, but it might get a bit too wordy.

Finally, I was supposed to move this guide over to the Official Payday 2 Modding Guide (and I probably will in the future) but at the moment I'm too lazy and have other things to do.

You will need the following, assuming that the Payday Mod Tool is already installed:

• Notepad++[notepad-plus-plus.org]

We will be opening a few XML files with this.

For this tutorial, we will be swapping in the 'arms crossed' animation.

1. Open up the 'extract' folder in Payday 2's 'assets' folder. Then, go to anims/units/enemies/cop:


2. Right click on 'civilian.animation_states' and select Open With. Find your Notepad++ icon and hit OK. You should see this:


3. Find the line that says the following:

• <to name="cm_sp_stand_arms_crossed" redirect="civilian/idle/group_1/blend_3/1"/>

I've bolded and underlined some text to point out what's important.
cm_sp_stand_arms_crossed is the name for our 'arms crossed' state. This line essentially changes or redirects this name to civilian/idle/group_1/blend_3/1.

Hit CTRL+F, and type 'civilian/idle/group_1/blend_3/1' (no quotes) in the box. Hit next until you find a block of text like this:

• <state name="civilian/idle/group_1/blend_3/1" type="once" segment="base">
  <anim name="ntl_irp_1_idle_b3_var1"/>
  </state>

Our underlined text, civilian/idle/group_1/blend_3/1, should be the state we just searched for.
The bolded text, 'ntl_irp_1_idle_b3_var1', is our animation name. Keep that handy.

4. In the same folder, open up 'civilian.animation_subsets'. It should look similar to the image above.
Remember our animation name? Hit 'CTRL+F' and search for 'ntl_irp_1_idle_b3_var1'. You should find these lines:

• <anim name="ntl_irp_1_idle_b3_var1" file="anims/shared/unarmed/neutral/ntl_irp_1_idle_b3_var1" blend_set="all"/>

Pay attention to this line:

• file="anims/shared/unarmed/neutral/ntl_irp_1_idle_b3_var1"

That's the filepath for the 'arms crossed' animation.

5. Go to the filepath specified in the '.animation_subsets' file, which is 'anims/shared/unarmed/neutral', and find 'ntl_irp_1_idle_b3_var1.animation.'

Make a copy of this .animation file in your Modding Workspace, which I'm sure you've made from the Modding Guide.

Now that you've found and copied your animation, it's time to find out which animation to replace for the menu.

6. Now, go to units/menu_character/player_criminal.


Open up 'menu_criminal.animation_subset'. Unlike the civilian animations, the menu animations are easily identifiable by their name.

Let's look at the shotgun animation line, since it only has one animation variant:

• <anim name="ntl_idle_shotgun_1" file="anims/shared/special/criminal_idle_shotgun_1" blend_set="all"/>

As you probably figured out, the bolded text is our filepath to the shotgun animation.
Add it to your destinations.txt file, but append '.animation' on it, like so:

• anims/shared/special/criminal_idle_shotgun_1.animation

7. At this point, you could be perfectly satisfied with only replacing the shotgun animation with your custom animation. However, this probably isn't the case, since you want everyone in your lobby to have their arms crossed, or dancing, or something else.
Instead of replacing every menu animation with the arms crossed animation, there's a much more simple way.

In our 'menu_criminal.animation_subset' file (I'm sure you didn't close it already), we're going to point every animation to the shotgun's animation, which is 'anims/shared/special/criminal_idle_shotgun_1'. Edit the file accordingly, it should look like this:

• <anim name="ntl_idle_menu1" file="anims/shared/special/criminal_idle_shotgun_1" blend_set="all"/>
  <anim name="ntl_idle_menu2" file="anims/shared/special/criminal_idle_shotgun_1" blend_set="all"/>
  <anim name=.....

and so on.

Save a copy of this file in your Modding Workspace.

8. At this point, you should pack your animation into a mod. I'm sure you're experienced with this if you've made it this far. But if not, refer to the Official Payday Modding Guide once more.

Your end result should look like this:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=253656369

Congrats! You've successfully replaced the menu animations. Wasn't it worth all of that work?
Probably not, unless it was funny.

Question: I can't see my character after swapping the animations!
Answer: The animation you're using positions the character off of the screen. Try using another one.

Question: My game is crashing after doing this!
Answer: While I haven't experienced any crashes while swapping animations, I'd recommend checking the 'menu_criminal.animation_subsets' file again to see if you messed up something.

Question: This is useless, why would you want to do this?
Answer: So you can get a laugh out of them. Heck, I learned how to mod these animations by making Chains dance.

Thanks a lot to:
I am not a spy... for documenting Payday modding and keeping the mod tools updated every patch.
Zwagoth for making the mod tools.
たつと (Tatsuto) for documenting Payday modding and keeping the modding wiki up to date.
[FTCN] Sam Wolf for helping me edit and write this guide.
AJValentine and T$ECore=ZEROALIAS=THT= for making a modding community and keeping it alive.