The most important part in building your skill set is self reflection; you will need to make honest assessments whether you are capable of developing an expertise with a skill. If you cannot effectively learn a skill, you will need to drop it from your skill set and decide how it may affect your career aspirations; although, there is something to be said about determination. Otherwise, you should understand your level of expertise with a skill in terms of what you can do with it – personally I find a level of expertise expressed in years as a poor measurement, though it is commonly accepted.
There are two things you should have from the planning stage: a list of skills in your skill set and an action plan on how to gain experience with each skill. I recommend having both handy. As you follow steps on your action plan, write a quick self assessment about how well you accomplished the related skill(s). This diary will not only help you keep track of your progress in developing a skill, but also be useful when you decide to present experiences on your professional website (and resume).
Keeping a diary might seem myopic in a way, but it is really not too overbearing. I will use my own use of a skill set as an example. I am a mid-career professional and have experience primarily in web and application design and business management; my whole skill set is comprised of 13 skills, 6 of which I use on a regular basis. It is simple to keep track of these skills in my every day experiences.
Sometimes developing a skill requires a team and it is always beneficial when you can get constructive criticism from your peers; sometimes you can get over the hump by responding to their feedback. For example, the skill of presentational speaking can be one of the most difficult skills to learn if it does not come naturally to you. Though it can be learned by practicing over and over again in small groups.
Another common way to get feedback regarding your development of a skill can come in the form of an assessment. This is popular in the educational careers. Students are required to get assessments when they are doing field assignments and these assessments are later used when they apply for teacher positions.
To summarize, during the building stage of establishing your skill set, you should follow your action plan to develop skills in your desired skill set and evaluate how well you are learning each skill – through self-reflection, team feedback, and/or assessments. Keep a diary of how you apply the skills in your experiences and your overall level of expertise with each skill. Be ready to drop a skill if you are not capable of learning it properly.