> nothing on how best to prepare the existing dog
I'm not sure there is a long list of routine things you can do to prep your existing dog as in most cases it requires adapting to individuals AND there's not a huge amount you can do in advance but here's a few thoughts :)
Sometimes it's about adapting your routines and your spaces to allow an extra dog plenty of 'room'. Room to eat, room to train, room to sleep etc without taking anything away from Meg. Does that make sense? You want to create some gaps
before the new dog comes so that Meg doesn't feel like she's losing anything to the newcomer. For instance, if you currently spend all your free time with Meg then she'll notice the puppy taking your attention away, if Meg is currently allowed upstairs but that isn't practical to continue when the puppy comes then stop it long beforehand, make space on your sofa for both of them, or none of them. Add the extra beds/crates in advance. If puppy training is every Tuesday for an hour, then start leaving Meg before the classes start, and before the pup comes home. Get your baby gates in place early etc
Teach a few new instructions that you think will be useful to manage dog interactions in your home. It can be really useful to ask one dog to go one way as you instruct the other to go a different way so following a finger, or left and right is useful. 'Wait' is sooo useful in a multi dog household - try feeding one treat to Meg and another to your husband/the fridge/a chair etc - no I promise I'm not mad! Meg needs to know that not all the good stuff is coming to her and she needs to know that 'wait' means it's not your turn, yet.
Talk to your chosen breeder about Meg way in advance, a good breeder will help you chose the right puppy to complement her, and will be willing to provide additional support if you need it. A good relationship with your breeder will also mean that you can get puppy smelling items each week to bring home so that Meg can get familiar with a new smell.
Not every dog will approve of a new puppy - some adults really don't like them much and find them a bit alarming. I know I've talked about my middle dog before (have a rummage for my posts if you can) he really isn't comfortable with other dogs getting too close and he took a while to warm up when the new puppy came along. He had incredibly complicated rules about what he would allow, where and when - even to the extent that his acceptable play circle was app 8 ft across and only in one spot in our garden. Prior to that my nitwit of a 28kg gundog would take refuge
on top of the garden table and pretend he was unconcerned by the mini hooligan that had arrived in our midst. He's now 5 years old and has only just started choosing to sit next to the puppy on the sofa ... the
puppy is two years old!!!
Patricia McConnell has a good book about managing a multi dog household. Some of her advice is aimed at trouble shooting an existing problem but there is a lot in there that you could use to prevent a bad situation from developing. Might be worth a look and it's a pretty short book, more of a pamphlet! Hope this helps :)