Give Her Specific Tasks
Chances are the mother of the groom is going to text you, call you, stop by in person and ask how she can help out with any and all things relating to the wedding. Instead of having her take on some tasks you'd prefer to do yourself, give her a list of things she can take ownership of and make her own, so she doesn't accidentally (or purposely) step on your toes.
Ask her to come along for shopping
First of all, settle any disputes between your mother and mother-in-law before you invite both of them for a dress fitting or any errands.
Including your in-law in the wedding planning makes her feel special and respected even before you walk down the aisle.
Put Her in Charge
Are you having a hard time dealing with a particular vendor, put your in-law in charge and have time to focus on other aspects of the wedding. Giving her power will make her feel involved and proud.
Let Her Know Your Non-Negotiables
It's your wedding day and as much as you want to involve your in-law, she must know her limits. she can't make any decision without seeking your approval. She should act within her limits.
Pull in a Detail From Her Wedding
Tradition must go on. Communication is key if you want to have a great relationship with your in-law. Ask if some family traditions must be duly followed and put her in charge to perform that task.
Let Her Pick Out Her Dress
It is her son's wedding and she has to look stunning and confident in whatever she wears. Give her the go-ahead to select her style but you can recommend a fashion designer she can work with to achieve a great look.
You may want to keep control over what your bridesmaids wear, but let the mother of the groom pick out her dress.