If you are not using fresh corn, you can skip infusing the stock as above but I recommend warming it before adding in stage below.
As stock is infusing/warming, peel and dice the onion, potatoes and carrot and cut the celery in similar-sized pieces. All should be relatively small - I suggest making the carrot, celery and onion similar size to the corn kernels, and the potato slightly bigger to allow for slightly different cooking times.
Once all are prepared, melt the butter in a medium pot over medium heat and add the onion. Cook until the onion is starting to go translucent, around 3 - 5 minutes, but without letting them burn. Add the carrot, potato and celery and cook a few more minutes.
Add the flour and stir to coat the vegetables and absorb some of the excess butter.
Add the stock, cover and bring to a simmer. Reduce heat and cook for around 5 - 10 minutes, depending on size of vegetable pieces, until the potato and carrot are tender to a knifepoint (but not overly soft).
Add the corn and crab meat, mix through and cook a minute or two more. Add the cream, warm through a minute but do not let it boil then check seasoning before serving.