My apologies in advance if I'm teaching you to suck eggs. Always difficult to know where someone is coming from, but this is based on my own experience as a self-taught TurboCAD user moving to one of these flash new, parametric, sketch-based, 3D packages. I'm sure from odd comments made that there are some professional Solidworks users and the like out there who would think my comments a bit trivial, but there we are.

First, forget most of what you have learnt from 2D CAD. In general, this is like a power-operated pencil and drawing board. You draw lines of given lengths, join the ends, add circles and so on, using whatever drawing aids the package gives you. Then you add dimensions which reflect the lines as drawn.

F360 and friends do it differently. Think back of envelope sketching. I would have said back of fag packet, but you can't get the fag packets these days! Use the drawing tools provided (lines, rectangles, circles, etc) to roughly put in the geometry of the part you are drawing. Accuracy at this point is not needed - big difference from typical 2D CAD. Now, where you want points, hole centres, ends of lines, etc, to line up, use the "constraint" tools to do this. You can lock points together, force lines parallel/aligned, hole diameters equal, and so on. At this point, or maybe a bit earlier if it was appropriate, you can start adding in dimensions. But you are not "reading out" the dimensions you have drawn - you are putting in a dimension that forces the corresponding geometry to match that dimension. For example, you have sketched four holes in a component, and set each equal to the others. Add a dimension to one of them, and they will all automatically change to that value. Change your mind - you meant it to be M5 clearance, not M4 clearance - so edit that one dimension and all holes change to match. This ability to lock drawing elements together is very powerful, but it doesn't come naturally if you are used to drawing "the old way".

Once you have your "sketch", appropriately dimensioned, etc, you can extrude it to give a 3D component. You can now select any face of that object as the base plane for the next sketch - maybe a new, mating, component or maybe just another part of the first. In F360, for example, you would select "new body" or "join" when you do the extrude accordingly.

It does take a bit of getting used to, and of course there are many more subtle factors to take into account when drawing - identifying symmetries in the part so you only draw one half then mirror it, using the pattern tools for multiple features like hole layouts, etc - but for me, the big hurdle was that first one of understanding that you add basic elements to the drawing as rough sketches, then add constraints and dimensions to force the final design. Once you get this concept sorted, it is really useful to be able to go back to a drawing, make one change, and watch all dependent parts of the drawing (or subsequent assembly of parts) change to match.

Finally, it really does help if you can look over a more experienced user's shoulder while they talk you through an example - that's how I climbed the first part of the learning curve.