You normally plant blackberries around December or January, but now is a good time to plan where you'd like to grow them.
We used 8 foot treated 4x4 posts and installed 2x4's for top braces. My trusty side-kick Ed put the posts in, then helped stretch the wire. To stretch the wire, we wrapped one end around a 2x4 and pulled! While Ed did the heavy work, I nailed in fence staples to keep the wire in place.
I initially purchased four bare-root Rosborough blackberries. Whenever you buy plants bare-root (meaning they aren't in a pot) soak them in water first to help rehydrate the roots. You don't need to do it for long - I just put them in a bucket while I dig the holes. I planted the berries about four feet apart. Once planted I gave them a good soaking and then spread leaves over the ground for a mulch. You need to keep them watered the first year, but once established mine only need irrigation once a week or so when they have fruit and once a month when they don't. I use a soaker hose that snakes around them on the berm. They spread out and fill in as they age - often to places far from the original plant. You can actually dig up these wanderers and replant them. Most often I chop mine up and put them in the compost. Be careful doing this though unless you have an active pile - otherwise they will re-sprout and you'll have a new blackberry patch.
The best way to keep your berries productive and healthy is to remove the spent canes every year. As soon as you pick the last berry, cut that cane all the way to the ground. Blackberries fruit on second year canes, so the new ones that are sprouting and growing right now are the ones that will bear fruit next year. The old canes harbor disease and aren't going to flower well, so take them out! Be gone to the compost bin I say! Not pruning the canes is the most common mistake people make and is why many berry patches eventually just stop producing.