There are various points you can begin planning a build and locking in the two key fundamentals, being the actual donor bike and the design you want to achieve.
Alternative A - Already have a bike, know the design you want
Planning will focus around mapping out the timeline for the changes and deciding whether you want to take the bike off the road completely or make small changes in stages while riding in between. You will also need to consider whether you have the skillset to do the work. If not, your planning will need to either cover sending work out to third parties or learning the required skills prior to undertaking the work.
Alternative B - Have a bike, not sure of what changes to make or the style to build
If you are clueless as to the changes you might make, start with a Google search for custom bikes of the same type of bike you have. Once you have some inspiration or some ideas, it is time to solidify them.
Go back to Design Methodology and get your ideas on paper or a digital format. Work through that methodology to come to a final design. If you're an Academy member, dive into the design video course in the membership.
Once you have reached this point, you’re now back to Alternative A above.
Alternative C - Don’t have the bike, but know what bike you want and changes/style you want
You first task is to obtain your bike, obviously aiming for the best quality version of the bike you are after that that falls within your budget. Regardless of the style of bike you are going for, you will want a mechanically sound bike. There is more detail in the book section BUYING A DONOR BIKE. If you know that you are going to make significant bodywork changes then you could accept a donor bike with degraded bodywork, or damage to parts that you know you are going to change anyway and save some money. Now go back and read Alternative A.
Alternative D - Don’t have a bike, but know the bike you want, but not sure of the changes/style to build
Work through Alternative C (don’t go to Alternative A as noted in that text), then Alternative B and finally Alternative A
Alternative E - Don’t have a bike, don’t know what bike to get but know what style of bike you eventually want to build
Your decision on what type of bike is probably influenced by a few factors. Your overall budget, whether this will be a recurring hobby and your skillset or experience.
For inspiration on what type of bike you should choose, do some Google research on what bikes are often used as a base for the style of bike you want and aim to select the donor bike make and model.
Now move to Alternative C.
Alternative F - Don’t have a bike, don’t know what bike to get, don’t know what type of changes to do or style of bike, just want to do a cool custom bike
This in some ways can be the most difficult place to start at, but in other ways also liberating in that you have all options available. However, a starting point must be picked and it's really either a choice of choosing a donor bike and then deciding what changes or style to go with or selecting a style and finding the bike from there.
It is probably simpler to decide on the style of bike you wish to go for and once you have decided that, go to Alternative E.
If you wish to start with a bike, it would be best to choose a budget for your bike purchase (ensuring you have a good budget left for parts) and then start seeing what bikes regularly come up when searching in that budget range, keeping some of the points from Alternative E in mind when selecting a bike. Once you have your bike jump to Alternative B.
The result of the above process should be a bike in your garage and a well-defined sketch or image of the final bike design.
Project planning would then continue from that point, fleshing out tasks, sequencing and durations. Then it's time to kick off your build!