How do proceed with larger electronic projects?
The general steps to the engineering design process are:

Define the Problem

This is typically the customer's or the boss's job. To decide how to fix something, you must first know what needs fixing.

Do Background Research

Understand the problem. Find out everything about it. Consult experts, literature, and conduct design reviews and generate questions for the client.

Specify Requirements

Take the customer's problem, and the information gathered, and put quantifiable specifications and constraints on the solution. This isn't a time to define a solutioin, only to determine how to measure the solution's success.

Create Alternative Solutions

This is where you take the specifications, and generate different ideas to approach the problem. For example can the problem be solved with a few logic gates, or is a microcontroller necessary? Customer input will likely be necessary.

Choose the Best Solution

Evaluate the pros and cons of each proposed solution. Choose the one that makes the most sense. There are many variables in that decision, such as price, availability, time, etc.

Do Development Work

This is where you actually implement the chosen solution. Simulate and breadboard until your heart is content.

Build a Prototype

Take the developed solution, build it, and integrate other aspects of the project (ie. mechanical).

Test and Redesign

Take the prototype and evaluate it against  the specifications. Make any design changes based on information gained, and complete another iteration of this process.