For those who are new to MMD model editing, here I've written down some general rules that apply to almost every MMD model out there. Sure there are some exceptions, but for the most part these rules will stay true no matter what model you're planning to edit.
First and foremost I just need to say READ THE ATTACHED MODEL README. I personally read the readmes for every model I feature on this blog (so I can write down the general editing/redistribution rules) but even though I do that please bear in mind that most readmes to Japanese models are only available in Japanese. I am not a native Japanese speaker so I could have easily mistranslated some of the information. Also, as models get updated so does their readme's; Even if an older version of a model might have some rules, a newer version could have different rules. And even if I say it's OK to edit/redistribute a model don't take my word for it and always make sure to read the readme yourself.
Now when that's out of the way, let's go ahead and jump to the rules:
1. Always credit
This is kind of an obvious rule, but still always make sure to give proper credit to the creators of the model/parts/textures you're using when making your edit. Preferably make your own readme when distributing an edit and add the credits there. If you're not planning to redistribute just write down the credits wherever you post the image/video of the edit.
2. Don't redistribute the original model
If you haven't made any changes to a model (not added or changed any parts/textures/facials/physics etc) then do not redistribute it, but instead link to where the original model can be found. Even if a model creator allow redistribution of an edit, it's almost always not allowed to redistribute an unedited model.
3. If you redistribute an edit, add the original readme
A lot of creators have it as a rule that you should add their original readme together with your model edit if you're planning to redistribute it. Even if it's not a rule though it might still be a good idea, as a way to let other people know of the original rules or as a way to credit.
4. No commercial use
Almost all creators forbid commercial use of their models, whether it be the original model or an edit. What this means is that you can't make an edit and then sell it for money or any other kind of currency (such as DeviantArt points). If you would like to use a model/edit for commercial purposes always make sure you got the original creators permission before doing so. For more information about commercial use, see here.
5. No R-18
Most model creators don't allow their models (or edits of their models) to be used for R-18 purposes such as gore or porn. This means both that you can't upload pictures/videos of the models in those kinds of situations, but also that you can't make naked (or almost naked) edits out of the models. Keep it classy ;P
6. Look out for additional rules
A lot of model creators, even though they allow editing and redistribution, might still have some additional rules regarding their models. Examples of such rules could be;
- Do not change the character into another character
- Do not take parts off the model
- Do not add parts from other models (even if they allow editing)
- All new parts added to the model must be made by yourself
- Do not make any visual changes to the model (so only edit bones/joints/physics...)
etc etc. So, in short; make sure to read the readme.