Are we talking an oil infusion or a water infusion?
I assume oil, in which case, a warm infusion is much cheaper, faster, and drastically reduces the risk of getting the icks in your oil. I used to make a warm mullein infusion for my kids' ear infections using the method, and it works just as superbly in soap. Fill a jar with calendula flowers, fill again with oil, within a half-inch to the top, screw on the lid VERY tight, and put it in your crockpot. I can fit a 1/2 gallon mason jar in my oval-shaped crockpot on it's side. Cover it with water, and set the crockpot to low for a few hours. Switch to the warm setting before bed, and leave overnight.
You need to warm it very gently or you will burn the flowers; it's a huge bummer to end up with crispy-fried flower oil!
Strain with cheesecloth; the last bits in the flowers that you really have to wring out tend to be the most powerfully infused, so really wring to get the very last bits!
I've done nettle infusions, calendula, rose petals, chamomile, and green tea for soap. All turned out good soap that I was proud of.
If you are willing to take a bit more time on your soap, it's very rewarding and much less expensive than buying ready-made oil infusions. They tend to be shockingly, offensively expensive.
***Make sure you have that half-inch or so at the top, a little more room with a crockpot infusion is okay, too. Oil expands when heated!