I get asked about mastering all the time by artists who want to record their own album. People with DIY studios want to do it all and they know mastering is one of those steps. There are basically four step to finishing a song; Writing, performing, Recording, mixing and mastering. All of these phases are a different mix of Creative vs Technical and they go from more Creative to more Technical as you move along.
The earlier phases are more accessible (i.e. they require less formal training, so long as you have a strong vision and sense of aesthetics). By the time you get to the Mastering phase, the technical side is dominant. If you don't have the technical training and proper facilities, there is no way you can master a track to "professional standards". Therefore, you shouldn't try. You'll be wasting your time and probably money since you'll buy (inadequate) gear that you (mistakenly) think will make you a professional mastering engineer. It comes down to the old 80/20 rule aka the "good enough" rule... (in short, doing a job "well enough" takes 20 percent of the effort/money of doing a job "right"). If you're not willing/able to hire a professional or to learn to be a pro (which will take years), don't obsess about it. Just do a good enough job to get your point across and keep going until you have the means to do it right.
I'm in no way discouraging people from self publishing! This is targeted at people who are primarily artists and dabble in audio engineering. Do we have the time and energy to both grow and improve as artists and to grow and improve as mastering engineers? For most of us the answer is no. So putting time and money into token improvements in our mastering skills and gear is probably a waste, since if we don't make a continued, focused effort on improving that skill, we won't see a significant improvement (or at least not significant enough to justify the time and money spent). So for us, doing some basic "mastering" (a little compression/limiting, eq, normalization) is "good enough" until we're in a position to pay someone who has put audio engineering higher in their priorities.