Frustrating as it is you probably won't find a useful step-by-step guide that exactly meets your needs (I know this because when I first started out in PM I also wanted to find one). The truth is that every project is different and you will need to adapt your methodology to meets its needs. For example, whether you design, wireframe or develop first will be determined by a number of factors: what does the client want/need; how much time is available to complete the work; how complex is the project; how much uncertainty exists in the project etc.
My advice would be to analyse an upcoming project and document what feels like an appropriate and workable project management methodology to achieve it. The five process groups from PMBOK are a useful guide to what your methodology needs to cover though the way you do it (given the scale we seem to be discussing) will probably be quite different. There's lots of advice on approaches to managing one-person projects elsewhere on PMSE that you will probably find useful. The best thing to do is give one methodology a try and stick with it for the duration of the project (unless it's going really badly of course!). If, at the end, it didn't work then change or adapt it for the next project. I assure you that this is how most PMs actually built their knowledge and skills.
Some specific advice
If you're consistently doing more work than expected then you need to think about how you manage scope. If you document what is going to be delivered (as functional requirements, users stories or whatever) and get the client to sign off on this then you've got your scope sorted. If the client then wants to change that scope (by adding more features etc.) then you can agree on additional schedule or resource costs as appropriate.
If you don't know where to start with the work itself then a good thing to do is to ask the client what they expect. Do they want to see designs first or are they more interested in a working prototype? Be realistic about what's achievable in a given period of time and agree how sign-off will work (how many rounds of amendments you'll make etc.) These kind of agreements are a fundamental part of the project initiation and should provide guidance to both you and the client on how the project will be managed.