I find that the tomatoes and vegetables cook down, and I do not want watery salsa. So when I am putting the salsa into the jar, I use a spoon with very tiny holes that the juice can drain out of so that most of what goes into the jar is actual salsa. When I get to the very end of the salsa, I pour it into a colander that is mesh with tiny holes. I place the colander over a pan to catch all the juice. I put all the salsa that is in the colander into a jar, and if it needs a little juice, I pour some from the pan into the jar. Then I throw the rest of the liquid in the pan out.
While I am preparing the salsa, I put the jars that I have washed out into the water canner with water that I heat to boiling. The water needs to be boiling when you put the jars filled with salsa into it, and this also sterilizes the jars.