Depending on where you are with your skillset, I would advise that you have a go at carving a spoon first as the carving techniques are much the same.
However, if you're experienced in spoon carving then happy days. A Kuksa is kind of like carving a really big, if slightly odd shaped, spoon!
Wood selection was the biggest challenge that I faced. I don't have ready access to green wood and usually hunt around for wind blown or any forestry works up my local woods.
Buying a blank is a good practical solution if wood selection is also a problem for you. But, personally, I'd invest more time in searching for a piece of wood because ax'ing out the blank is a big part of the fun.
The advice above about using a curved gouge is good. I use a spoon knife and it took a good while and left me nursing a few blisters on my soft office worker hands! You can only work with what you have though.
Spare a thought for what you want to be drinking out of the cup. Spirits, wine and espressos won't need so much volume and will be easier. However, a nice cup of tea might need a bit more space!
Be aware that hot drinks can cause problems in wooden vessels. Cracking can be a problem. To that end it might be worth ensuring that your bowl is a little thicker than you might first have imagined. I'm far from an expert here though, others might be able to offer some more wisdom on mitigating this issue!
The best bit of advice I can offer you though is to just take your time, rest those tired hands when necessary and enjoy the process.
