As stated in the other post, make sure you are taking cuttings from green branches and not woody branches. Woody branches will root, but will be stunted. Please read that post.
Always make sure to put at least an 10' tree pole next to the plant once you've planted it in the ground. These plants get very top heavy. I've had 12' plants that were tied to tree poles fall over in strong winds when the ground gets really wet during the Winter. Ideally, the tree pole will have three feet in the ground. This only matters if you'd like to grow the collards upwards of 10'. If you continue to prune the main trunk AND the side branches, you can keep it smaller.
(Please aid the plants survival and propagation by sharing cuttings with other gardeners. You can send them here for directions to rooting or better yet, root them first.)
Side note: Unless you are taking cuttings to make new plants, you'll always be harvesting just the leaves.
I have grown plants in two formats.
1) I let the plant grow to about four feet plus and then top two feet off and root that cutting. Once the apical meristem is removed (the top of the plant) the plant will immediately direct its energy to the lateral meristems (lateral branches). These branches can will then provide you with more opportunities to make cuttings. I will cut all the lateral branches when they get about 4'. The plant will now start to get up to 4' in diameter. You can repeat this process at least one more time. Growing in this manner, the leaves will be smaller than in growing them in Format 2, however, you will have a lot more production.
2) I let the plant continue to grow without topping it. Somewhere around 6' tall, the plant will start putting out lateral branches. You can eventually make new cuttings from these branches. I prefer this method because I love seeing a 10' plus high plant with GIANT leaves. However, (as you can see by the two photos attached), this three year old plant with one stock has just had all of it's lateral branches pruned. You can see the woodiness to the branches. These branches will produce quite small leaves and you won't be able to make new cuttings because of the woodiness.
Rooting the Cuttings
Make a cutting with at least 5-6 leaf nodes. Strip all leaves off the cutting except the top smallest leaves. I dip the end of the cutting in a rooting hormone, although this isn't necessary. My neighbor has just stuck them in the ground (in Spring and Summer) and some root and some don't
The medium I use is 50% perlite and 50% vermiculite. I use a 4" pot. I then cover the pot with a plastic bag to make a mini greenhouse and keep it in total shade (Summer) or indirect light for the next two months.
They take a really, really long time to root!
This is why you can't find them at nurseries; there's no profit margin given the amount of time and energy it takes to make new plants.
(It's possible that your cuttings will flower in the first three months if the weather is warm - like late Spring or into Summer. Unfortunately, you'll have to start over. I think it has something to do with them being in the pot - possibly a bit root-bound -)
Be very careful when removing the plant from the 4" pot as the roots are very delicate and they are growing at the very end of the stem cutting. These roots can fall off easily if you're not careful. I like to let the soil dry out completely before popping it out of the ground. I have often transplanted them up to one gallon pots before putting them in the ground.
Give them a good feeding of Calcium every month or two.