I used to use the original cut and paste technique. Print out a picture of your bike. Print pictures of tank, seat or other feature that you like. Cut out that tank/seat etc and literally paste it onto the picture of your bike. Simple as that.
I later graduated to two different versions of CAD.
Cardboard Aided Design for design of components and
Crayon Aided Design for trying different color schemes and simple shapes.
I still use all three, but if I want to develop a different look I use Microsoft Image Composer which used to be part of the Front Page suite years ago and is probably available as a download somewhere. It is not a fully featured photoshop or similar product. It's a very simple tool that allows a user to makes simple changes to an image.
It's not a print ready product but an easy way to add shape to sat a seat hump in stages and save each iteration for comparison.
But most of the time I just print out a picture of the bike and use pens or pencils or crayons to add shapes, change colors etc. If I have to remove part of the background or say a feature on the bike, I just use a plain white shipping (or other) label and then draw in the new shape.
Pros: low cost (zero)
Short learning curve (zero)
Cons: To "save" an image it has to be scanned in.
Not exactly high tech
Keep in mind that digital tools are just trying to copy what we all used to do before digital was even a word.