Get your outboard's HP rating and shaft length. The outboard's shaft length will be essential to determine your bracket's mounting height, and your engine's power versus your hull's weight/load will allow for you to select the correct bracket setback distance. Typically, for a heavier/longer hull and low HP combination, a longer setback (e.g. 30" or more) is best. For a lightweight/short hull and higher power engine combination a shorter setback works better. Determine your bracket's setback distance. Determine your bracket's mounting height (see this video for single engine brackets, and this one for twins), which will allow you to select the right swim platform's width, and make sure your selected bracket's airbox height will allow for at least a 3.5" step to occur from the edge of your keel to the bottom of the bracket's airbox. Determine your swim platform's width. Measure your transom's vertical angle.