You no longer need to edit your post #1 and add [Solved] to the subject. (DON'T DO THAT.) The new "solved" forum feature adds a button with a checkmark icon next to all of the replies in your topics, so what you do now is click on the checkmark button next to the post that gave you the solution, or next to your own reply if you solved it yourself. This will cause the entire topic to show as being solved. If you solved the issue yourself or found the answer to your question by yourself you should also post a summary of the solution or answer for the benefit of others.
Here are the old buttons prior to the feature addition:
and here are how they appear now:
(Sorry about the different browser zoom levels when i made the two screenshots.)
The tooltip that appears on mouse hover says "Accept this answer", the idea being that you click the button next to the post containing the actual solution. That way when others read the topic they know which of the many suggested solutions was the one that actually solved the issue.
If you later find that the issue isn't actually solved after all, simply click the same button again to remove the solved status from your topic.
Note that only the person who started the topic (and the forum moderators and administrators) can mark the topic solved or unsolved: no one else will have that checkmark button. (Don't worry, the mods aren't going to go around marking peoples' topics solved except in rare circumstances, such if the original poster (OP) contacts them and asks them to for some reason.)