This answer is "you shouldn't", since it appears from your description it's not your land and you have no authority over it.
Unauthorized private modifications to public land can be a serious problem. Not only can you get into trouble doing it, it may make work and cause problems for those who are in charge.
If you really want to see this trail marking changed, find out who owns the land and who has authority over the trails (may not be the same people, committee, etc), and make a request to them. When doing so, make it clear you are willing to help as a volunteer to perform any work. However, you should not take it upon yourself to make modifications to something that's not yours without the proper permission.
I can tell you for sure that if you came to my town and started modifying markings at trail intersections, we'd take a very dim view of that. If you came to the Trails Committee instead with a proposal, we'd kick it around and evaluate the idea with the benefit of the general public in mind. There may also be various restrictions and deals made about trails on that property that you are not aware of. Those may proclude a certain type of use, or even a certain type of signage. Yes, both of these restrictions, and a number of others, exist of various parts of our trail network.
Added in response to comments
Wow, we have a tough crowd here. The main objection seems to be that this answer doesn't apply to the OP's location. Maybe it doesn't, but there is no way to know since the OP never mentioned a location. If you want a answer that is specific to a location, you need to specify that location in the question.
I just checked, and the OP is in Bulgaria according to his profile. I don't know what laws and customs are like in Bulgaria, and of course we don't know if that's where the OP is actually asking about either. In any case, this answer is still correct for significant parts of the world, including where I am, which is in the US, more specifically New England.
Around here, even in a city, someone will own the land and have authority over it. While walking on it is generally OK as long as it's not specifically posted "No trespassing", making any modifications without permission of the land owner is not OK. This would include trimming branches to make a trail, or to deliberately obfuscate a existing trail. It would absolutely include putting up a swing and installing benches.
To take this even further, in some cases modifications, even just clearing a trail, may take more permission than just the land owner's consent. There can be various ordinances in a city governing vacant property. Here in Massachusetts anything within 100 feet of a wetland requires permission from the local Conservation Commission in addition to consent from the land owner.
So all those quick to downvote because they assume it doesn't apply to the OP's particular sitation need to consider that it does apply to many such situations in many parts of the world. I don't really care whether this answer ultimately helps the OP or not. However, it is something anyone else contemplating something similar should consider carefully since it may apply to them. Downvote as you see fit, but I'm leaving this here because it is in line with the Stack Exchange stated goal of building broad and long-lasting repository of questions and answers.