It may be tempting to handle some aspects of the work yourself (after all, it can give you more control over the project and/or materials) but if you wish to do so then make sure you discuss it up front. Many builders prefer to only take on projects for which they do all the work from start to finish, but some are more flexible. Go down this path very carefully so you are both clear about what the builder is responsible for finishing and which elements of the project rest with you.