Nipple stimulation, as covered, is a starting point.
Try this resource
https://milkology.org/content/how-to-induce-lactation-without-pregnancy
Then, once you start to get initial supply, something like this can increase the quantity/quality
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If you have more questions, feel free to ask, this was a favourite thing of mine. Actually, with one partner in the distant past. My current partner breastfeed both our children for nearly 6 years in total (3 years each & me, any chance I got, lol). We don't practice this anymore, but definitely can be a fun activity.
In practice, we used it in breast bondage, humiliation, and all manner of bedroom activities.