Hi & Welcome to this awesome group.
My rescue/rehome girl came with +food guarding, alot, plus possessiveness, and jealousy.Seriously.
I used basket muzzles in early training to avoid injuries, but I could also reward GOOD behaviour through these via the side. They all learned to take treats introduced through the side.
All of these traits went to lack of proper training from a pup, growing up around other similar (untrained) minded dogs, and possibly ?SA and/or insecurity.
I started training when I got her.
She was two and a half years old, two weeks post spay, and wearing a cone.
My boy Chester had come at nine weeks, and was just a few days' shy of his second birthday.
As he kept 'jumping' her, he went in to be neutered, too, three days after Eski came, so both were in cones.
I had the free E-Book downloaded from Successdogs.com and started with Sit, on both, with a small treat visible in each hand.
(They are both food oriented).
● Rewarding within three seconds IS IMPORTANT.
(It takes 30 repetitions to get a new command 'IN' and repeat it several times, daily).
A treat for every good response.
Ignore the bad, praise the good, incl this verbally, &/or with a clicker.
Just keep asking quietly, (and using a hand signal) until they get it, and praise verbally when they get it right : "Yes! Good dog! Sit" or, whatever you asked.
From Sit, we progressed to Down.
From here, try the "Middle" game (on my fb timeline), from Absolutedogs.com
I also did the hiding treat under foot... (successdogs.com) which gets them to focus ON you (eyeball to eyeball), not the treat on the floor.
This now works on both. My boy is much more keen to play, learn and earn a reward, my girl is lazy. She's learned much through copy-catting, (however, KNOWS very well when she's done wrong, (ie, chewed up stuff ready to go round to the back bin/s), and shows absolute submissive and (comical) apologetic actions!) And she knoes I know she knows what to NOT do. She's very argumentative and vocal - well, Chester is too when I come home, & foster boy Blu (came Nov 25/2018) is now 17 months, nearly 18 months is just finding HIS voice. He has the most to learn, coming from one owner, increasing day hours in a 12' x 6' kennel and no polite respecting of pack etiquette, or around more than one hooman! Blu has a basically sound, sweet nature, however is over bouncy with excitement and learning hard and fast rules - which started* the night he came home.
I also worked on 'Leave it!' with them, separately. Then 'Give' ie, to let go of a toy in exchange for a treat/ reward (work on this separately first) then with another furkid if you have them.
Leave & Wait : (start with a three second 'Wait!') and slowly, day by day with the multi-repititions same day, increase the wait time each day.
● [ Wait! Is also vital when exiting the car, giving you time to unclip from in-car restraint, (UK 2014 law ref transporting all animals in your vehicle) and get your leads together/hooked on to canibelt. Also prevents escape, or running into traffic! ]
From there, 'Bed, Time out', important if their behaviour is undesirable; this also means rejection - in the relevant situations.
NOT meant when 'Bed' command is given, (to have them safely out of the way, into their crate).
Again, crate training is on successdogs.
Crate use is really important if you need your dog/s out of the way for visiting workmen, utility repairmen, dog shy visitors, and little humans and, infants FOR SAFETY and convenience.
I reward mine through their crates, when they go in for this reason, and praise too "Good dogs, bed!"
The "Bed, Time-out!" means rejection; ie, 'I don't want you here doing what you are doing!'
However, to do this, YOU walk them on a lead OUT of the room to their crate, OR into a separate room, into isolation, & shut the door.
All mine know that's for good reason, and know it is my 'rejection' of them due to their bad behaviour - ie, no treat when put into crate immediately the bad behaviour happens).
Leave in for three to five minutes.
Then bring them back out.
If they repeat (any) undesirable behaviour, (You can say "No" and (silently) clip on lead and walk them back out again.
WHEN they return, and ARE being 'nice and good', praise enthusiastically, AND reward "Good dog! Be Nice!"
You both - ie, couples - need to work together on this, so they grasp where their place is in the family human pack. Young children can learn a lot just watching, as can other furkids in what you do. Older children need supervision if getting involved re training as positive reward training MUST be used always and only.
[[ Shouting or striking -> fear. Fear -> defence -> aggression -> a snap, or worse, a bite, and, ultimately an attack if a strike (for a scolded action is given) and, human body /verbal language also triggers fear. ]]
It won't all happen immediately re the behaviour you seek. If they get it wrong, or don't get it, go back to basics. It's not their fault. It is yours. 😐
You need TIME, patience, perseverance and planning (ahead) to achieve all these.
Go to my timeline (videos) (some appear now to be mis-named, however you'll find my earlier and later ones showing how I worked.
I use mushing terms when out walking, plus "Here" frequently to get them to refocus on me and turn round, come & sit in front, Sit &/or Down, or Paw on my lifted knee for a treat. Especially if I see another dog in the vicinity. Most folk cross the road, or, I move off the path if I can, to increase distance.
When I can't walk out due to discomfort/pain in my neck, (severe crumbling bones to wear & tear) I resort to my mobility scooter.
They've learned to keep paws away from front/rear wheels, and they stay on my canibelt approx level with the front, altho' I can bring them back parallel to me, or, they move behind, (command: 'Whoa, Wait') to let me through narrower spaces first. I can cover much longer distances with them, on this, and at varying speeds. Tires them out beautifully!
Look at Absolutedogs.com too.
Good luck! 🤗