Starting to Supplement with formula

When you made the switch from exclusively pumping to supplementing with formula, how did you go about it?

My scenario: I'm 6mpp I have been Exclusively pumping since the beginning. However this summer I got Covid then then stomach flu then my menses came back. After each incident I spent so much time power pumping and working to get my supply back. I was able to get it up but while I was an over supplier I am now an undersupplier. With a bit of a freezer stash we are still able to meet LOs daily needs. Before I have to cold turkey switch to formula I want to start introducing it so I will know if LO is willing to take it.

I've searched how to do this and no one really discusses it. They talk about cutting out breast feeding sessions and introducing a bottle of formula. But that's not what I'm really looking for.

Do I offer him a full bottle of formula? An ounce of formula? Do I mix the formula with breast milk? Could I try the formula in the rice cereal?

Please help.