Best way is firstly, don't say anything along the lines of "I always hated/couldn't do maths". If Mum can't do it, then there's no way I will be able to!!
Games are often the way to do it. There are lots on line, but you can do ones yourself.
Treasure hunts. I've done those with a maths bias. Solve the question and it gives you one word towards the final clue.
Simple board games-you can do multiples as well as adding.
Challenges: I need you to go to the shops and get me X, Y, Z X they cost £x, £y, £z, work how much change I'll get from £5 and you can keep it.
Or I need to get as many packets of this for £1, how many can I get?
Recipes: Do a double (or half) recipe. Ask her to work it out for you.
We need to leave at 10:45, how long do we have?
You can lead her through things. I'm planting 9 potatoes, each plant produces about 4 potatoes, how would I find out how many I should have in total. What do you think it is?
So thinking about things logically. Just in discussion. eg I wonder how many blades of grass are in the lawn? Shall we guess? How might we estimate (count number in small area and multiply up). You don't need to do it (or do it together) but it gets her thinking about how to do maths, and if she can understand it then it will help with her confidence.
You can do similar things: How many books on her bookshelf, how many in the library? How many bricks to build the house?
Games like darts, where she has to add up the score.
We have a card game which is good for basic adding. My mum made the game up as a child and it's still my dc's favourite game. It's known as "Granny's game". I knew it as "Mum's game".
Take the picture cards out.
For round 1 you get 1 card, round 2 you get 2, all the way up to round 10.
You have to put the cards in groups that add up to the number in the round.
So, for example in round 6, you might win with
6, 3,2,A 5A
Round 10 you might have
10, 8,2 8,2 6,3,A 5,5
You start with the cards in your hand and pick one and throw one each turn. You can pick up the last discarded card, or one from the pile. Between each round you reshuffle the cards.
Main thing is to make her feel confident. If she's wrong, then take a look together. if she's right, then be pleased.