I just got my first sub and went through the same thing. it is, in fact, something you can do yourself. I am the sort of person who has difficulties tying my own shoe laces

. if i can do this, anyone can.
First, check the tube for instructional videos. there are multiple ones. i think i watched one made by Rolex or some high end watch retailer. Have a look at the process, tool you need (as said above a flat head screwdriver) and listen to the instructions. there are some tips on what links to remove and how to add links back if needed.
Check your toolbox. you may already have the required screw driver. I did not so i went and bought a cheap, really piece of c**p, watchmaker set

. this is what they call it but in essence it is a set of screw drivers which cost me a EUR1.10. Yep, a bit over a euro so you can imagine the quality

. once you have this, place your watch on an appropriate surface and unscrew the pins. Remove the link(s) and screw back. Careful not to scratch the bracelet. I did. No biggie, but still i could have avoided it if more careful.
Job done.