Congratulation on the new pup and welcome to Boxerworld.
Simple commands can be learned almost immediately. Things such as sit and down are easy commands to begin associating with words because the pup does these things voluntarily and often. When a dog volunteers a behavior, it is the easiest way to get the dog to learn that behavior.
Each time Aspen sits, as she sits say the command "Aspen, sit" and then offer praise: ("Good Girl, Good Sit!") very enthusiastically, and reward the dog with a tidbit of a treat (bit of hot dog, puppy biscuit, etc., something small but tasty). Same with the down command. The dog will in about 2 or 3 days start to display the behavior when you issue the command. Of course, that also depends on how often you work with your pup. I recommend several short (5 to 10 minute) sessions on one command at a time. And remember, all dogs are different and some pick up things faster than others. So, if you find your dog does well a few times and then seems to "forget," go back a step as it would appear that you've moved to quickly for the dog.
I always recommend a command be at at least 90% reliability before moving on to the next command. Then the second command and then combining the first two commands as reinforcement.
But you need to show the dog what you want (the voluntary sit or down) and then reward it to show it that it is a good thing, and then associate that behavior with the command word. Just saying "Sit" won't get it. You need to guide the dog into the behavior and associations you need.
One thing you need to work on is name recognition or "Watch Me" command as it is sometimes called.
To do this, each time you call Aspen's name and she looks at you, again enthusiastically praise her (Good Aspen, Good Girl!) and reward with a tidbit. Ideally, you want the dog's head to "SNAP" to look at you when you call her name (yeah, like that'll happen with a boxer

). But, the idea is that if you can't get the dog's attention by calling her name, you really can't get much further.
You might want to check out the book section here at Boxerworld and read about some of the books members recommend for training their dogs. Some very useful information there.
Good luck and keep us posted.