Hi Krissmeda.
When you are daydreaming you are visualising. When you are scripting you are visualising through the written word, even when you recall things in your mind you are visualising. 
Try this.. Close your eyes, and imagine yourself in one of the rooms in your home. You can visualise by either seeing yourself as if you are watching yourself, or you may visualise as if you are there looking out of your own eyes. Now in your mind walk to the fridge. Now to your bed. The toilet/shower, then the front door. Now go turn on the TV and pick up your keys from wherever you normally leave them. Once you have done this, head into your bedroom and look out of the window. What do you see? 
You see by doing this exercise, as often as you need, you are helping yourself tune your visualisation skills up. Whilst you can remember the layout of your home, then you are imagining and visualising. This could be done for any familiar place that you know. Once you have done home, maybe do work. Visualise yourself doing walking around the building you work in. Sitting where you normally sit. eating lunch where you normally eat lunch. 
I know it may seem a mundane exercise but it will help, and you will soon come to realise that you can visualise after all. We all can. If you have a certain thing you are trying to visualise then try starting at a mundane process. For example, you are after your new car. So you wander around your home in and out of a garden perhaps, and then into your car to drive to a garage. Or even you could open your garage after you have wandered about to find your car there. 
The more you practice it the easier it will become, and the quicker and easier you will be able to do this. 
To be honest though, if you are doing the other things, then you don't necessarily need to use visualisations. There are so many other methods you can try. The point I am trying to make here is 'don't stress it'. The law of attraction is fun, and so find something that you feel as comfortable as possible doing. I hope this helps.

Hi Eveee,
Thank you for you reply, it's really helpful.
You're right the process it's fun and should be fun. But I think, at this moment, I'm more aware on the fact that I want to visualize as to complete tasks on the 25 days challenge book and I took that as something that I have to do and left out the fun part of it ...because I usually fall asleep or my mind drifts to other things or goes blind when I visualize those suggested scenes.
That's kind of strange because  imagine a lot of scenes, not regarding to my SP but I do imagine and visualize daily.
I guess I have to train my mind and stop seeing it as a task that I must complete and start seeing it as a fun thing like the rest of things that I do.
I will definitely try visualizing familiar surroundings , it's mundane true but the idea it's to make it easy so... I've got nothing to lose.
I also have to  take it easy and have a break whenever I can't really visualize as in before sleep .
Thank you again for your help,
Cristina