How do I assign tasks specifically to dwarves

This is not something you can actually do; What you do instead is tell each dwarf what types of jobs to take, and it will pick its own jobs from all the jobs that need to be done. Here is an elaboration:

You should view a dwarf's preferences and labor, in order to see a list of jobs. Select which jobs you want that dwarf to do, and deselect the jobs you want it not to do. For example, if you want a dwarf to mine all the time, select mining and turn off all other jobs (like the hauling jobs near the bottom of the list that are always on by default.) Then that dwarf will only do mining jobs, and as long as you have designated some area that needs to be mined, that dwarf will take care of it.

Some jobs are generated at workshops. If you build a workshop for masons, a dwarf with the Masonry labor activated will put it together for you. Once the workshop is assembled, you can look at it with the q key, and select a list of things you want built. For example, if you add a request for rock doors to the list of things to be done at that workshop, a dwarf with the Masonry labor activated will come along and start making them. If you want him to do it sooner rather than later, be sure to deselect all his other labors besides Masonry (again, especially hauling.)

There is one way to get around this for certain tasks that originate from a workshop or other built structure. With a dwarven manager named, query a workshop and set its profile to a single dwarf. From now on, all tasks that are queued at that workshop can only be handled by its named dwarf, whether or not they have the appropriate skill activated at the time. Using the manager to queue a large job will assign jobs to random workshops of the appropriate type, but queued jobs are still limited to being executed by that shop's assigned worker only. Strange moods may disrupt this subtle order, but not permanently.

Tasks like Sand Collection from a glass furnace are free for any dwarf with the appropriate hauling skill activated, as is Capture a Live Land Animal from a kennel linked to the Trapping skill, rather than any profiled worker.