This guide is not gonna cover everything just some necessary basics mostly.
Now first step you want your Gu-Ku! its simple kinda. First go to the caretaker cat in your farm(The one in the back of your farm usually holding a broom and keep note her clothes change the more Gu-Ku you have) talk to her(At least I think it's a "her") and she will tell you that you need to get a Gu-Ku egg!.
Now to get the egg talk to the Guild Master(Yes the smoking overly relaxed short old guy} and he will give you 5 quests to pick from each quest has a monster holding a egg in a way and each quest has a different color Gu-Ku in the eggs.
Now for the quests to complete the quests you must deliver a Gu-Ku egg to the red box.
Quest 3: Quest 3 is in the Desert and the egg is with a Daimyo Hermitaur. Throw a sonic bomb at him while curled up guarding him self or Just follow it around without being spotted and it will drop the egg while eating.or leaving area. This quest gives a Black Gu-Ku.
Now go to the guild master(Yes the guy who sits around smoking again) and he should give you a quest to hunt down Melynx and deliver some papers they drop. If I recall it was 5 papers dropped by Melynx in the jungle. After you do that return to the caretaker cat and see if the nest has been built (re-enter the room and the nest should be built).
Now your egg should be in the nest walk up to it and pick any of 3 options to open up more options to play with the egg. I heard that the first 3 options can change its size when it hatch's after playing with it> 1st is for normal size, 2nd is for smaller and 3rd for large. You can play with it once after every time you do a quest and it usually hatch's after 4-5 quests. Remember also once you name the Gu-Ku after it hatch's you can't rename it so if you named it wrong you can always release it and get another one but you have to wait after doing so for like some days I think.
Now after your Gu-Ku hatch's talk to the caretaker cat and she will tell you to pick up the Gu-Ku(1st option when you press the Gu-Ku) and carry it to its nest to sleep. Talk to her again and she will tell you about feeding the Gu-Ku(2nd option on the Gu-Ku).
After that talk to the cat again and she will mention you can dress up a Gu-ku's if you want but guess what... she will mention that some Melynx stole her blueprints for the Gu-Ku swimming pool and dressing stuff so same as quest before just it should be in the Great Forest now(Sheesh she travels a bit too much).
Once you complete this return to the cat and re-enter after to reveal you now have a pool and sign to dress up the Gu-Ku. Now get yourself a second Gu-Ku and then you can breed them when the cat sometimes randomly asks you a "Yes or No" question. And if you ask no I don't know what colors to breed for results except I got Black and Blue and I got Crane Gu-Ku's.
Now once you have everything set you can mess around with the options at the sign to change Gu-Ku clothes, Nest, Pool, Floor, and at the house icon pick what items they have for you after each quest 1st is Fish, 2nd is Bugs and 3rd is random garbage that may come in use.If you talk to the caretaker cat her 2nd option is to cook with a Gu-Ku and she will ask you to pick a item to cook. The number next to the item is how many you cook for the resulting items on the right when over a item. Now this really shines at HR100+ items as for example if you are in bad need of LV2 Carapaceon shells but all you got is a bunch of LV1 Carapaceon shells you can cook 10 LV's to make 1 LV2 Carapaceon shell. It may sound like a ripoff but that depends on how much you need it.
Also its kinda like cooking Well Done Steaks if you time things incorrectly you lose the items you were cooking with so be careful.
Any questions and opinions are welcome..