How can I specify the position of the marked features ?
If you are talking about the position of the holes' centers, I'd just use the center lines of the holes with respect to the center line.
Regarding the 5.8, are you sure you need to give that one? Because in that scenario probably its best to give the centerline and countersink surface.
@r13 and @NMech have given you great information on how to reference dimensions relative to main features of a part: like the centerline of a hole or relative to key surfaces.
When you design a part and have it built, it may not look exactly as you envisioned.
Machining leaves tool marks, surfaces may have groves or ridges (waviness), corners may be absolutely sharp or deburred, and holes may have grooves along the interior.
What you specify to control the accuracy of your part affects the cost to manufacture it. Polishing a surface to a mirror finish is labor intensive and very expensive.
Simple dimensioning of a part is useful. Consider a washer for a bolt, for example: make it so thick, with an outside diameter, and an inside diameter. How co-axial, or how close the centerlines of the two-diameters coincide may not be important. If it is, you need to reference one diameter relative to the other. Consider also the allowable variation in the thickness if it is bandsawed from rod stock.
There were (are) courses in college that taught (teach) Drafting and many books remain available: both in libraries and on-line. Simple dimensioning was the staple for many years and eventually got us to the Moon.
This link downloads a good article summarizing (GD&T) Geometric Dimensioning and Tolerancing. Just look at the pictures of the download as they will give you an idea of what your finished part may look like, given your design specifications. https://d2t1xqejof9utc.cloudfront.net/files/18904/Fundamentals_of_GDandT.pdf?1363288254
GD&T has it's advantages and disadvantages. I have encountered machinists (manufacturers) that only see dollar signs (when GD&T is used) as checking the part with only calipers makes it more difficult to verify the part meets all your specifications. However, the proper use of GD&T helps the designer to make sure components will properly fit together and function as intended given all variations on the tolerances of manufacture for all the various components. I found it to be a skill that is developed over time: from design a part to go here and do this to as you begin to design assemblies and components that all interact together.
Talking with coworkers and seeking ideas or direction will help guide your designs, talking with manufacturers will give you insight into their concerns, and learning through them all will help you grow as an engineer.