It takes quite along time to get them ready to put out. I grew them in the UK and this is how I did it, it may not be everybody's choice but it worked well for me.
I started the potato off very early, it need a minimum of 12 to 16 weeks to get a big healthy plant that can compete with the UK weather, it was easier when I grew them In Canada before I came over there for 9 years so I adapted my Canadian method to suit the Yorshire weather.
The following is presuming those reading it have never grown them before.
As I don't want to put these out till the weather is warm enough for peppers and the growth speed is about the same as peppers too I use that date for working out when to start. It takes at least 12 to 16 weeks o get them ready for the ground.
Get a large jar and a large sweet potato, it may be called a yam, trust me there are no yams in the average UK supermarket what ever it says on the sign, a yam has a barky surface, you can't miss it if you ever do see one and they are from Africa. For your first try don't pick the variety that is purple and may be described as Japanese they are even more tender.
You will need some tooth picks to stick in the potato so they rest on the top of the jar with three quarters of the potato above the jar, the lower bit in water .Keep the water topped up and pop it on a sunny warm windowsill. I would start when I started my chilis in January, you can't really start them too early but you can start them too late. January gives you a good well established plant to go out when the weather is hot enough.
After some time roots will grow out of the bottom of the potato and green shoots that point upwards will grow, don't worry about the roots at the bottom it is the shoots we want, they are called slips. When the slips are about6-8 inches long..no sooner..snap them off at the potato. Get a new jar of water and put the slips in there. Now some folks will pot these, I don't. Let them grow on till the slips have formed good strong roots and then pot them on into 1/2gallon pots in potting soil and wait for them to grow bigger and establish a good root system in the pot. By this time they should be Ok in the greenhouse.If it gets cold at night bring them in or fleece them.
. When the weather is reliably warm, they can be planted out with spacings as I said earlier. They will rot if they get excess water when in the ground. They grow well in a polytunnel , I have grown them in very large pots, like half barrel size but you won't get the crop that you will in the ground.
Unless you have a really cold wet summer and the timing is right they will grow without problems.
If you start one too late, pot it on anyway it makes a lovely houseplant.
The secret is start early, pot on till in big pots and wait for the very warm weather then give them the proper spacings..and you are putting out decent size growing plants.
I was in the UK 9 years and grew them very year, the first year was not good when I used my Canadain way then I adapted the growing method. Bear in mind most of the net info is US and not accurate for the UK..this works.
Hope this helps anyone who has not had success or is new to trying.
XX Jeannine