The first thing to consider when designing your boom mount are the materials. Generally speaking, solid steel plating is required for almost any type of design. The plate forms the base of the boom mount and ideally it should span across your truck chassis for stability. Heavy weights will stress the boom and this stress is transferred to the mount, so it must be reinforced. Steel bars, channel steel and iron rods may also be utilized to reinforce the truck chassis. You will have to design the mount and bracing to be effective but lightweight, so as not to affect the performance of your truck.
The boom mount is as simple as welding a solid steel plate to your truck bed, and as complex as reinforcing the entire bed. The design depends heavily on the type of equipment you are using with the mount. Consider the maximum payload of your truck, and design the boom crane to meet this weight capacity. That ensures your maximum payload is achieved. The weight of the crane itself will count towards your total payload, so factor this into your equation.
The mount bracket which attaches your boom to the truck is another consideration. This is designed with the specific mounts you are going to use in mind. The mount must mate with the boom. Secure a set of dimensions for your boom in order to craft your mount correctly.
Building the mount is another process. The mounting plate is fashioned from solid steel using either a cold press or heat press process. Steel is bent to form a solid, stable mounting surface that mates with the appropriate boom extension.
If you have no experience in this process, team up with someone who does. Welding with a MIG or TIG welder is inevitably required, so these skills must be part of your repertoire. The welds must be made to be sturdy and strong as a snapped boom will caused severe damage to your truck bed and the payload, along with possible injury to anyone working around your truck.