R2D2 is the famous droid from the "Star Wars" franchise. He appears in every movie and makes only blips and bleeps to communicate, but remains an icon. With the Star Wars LEGO toy blocks franchise, he appears as a small figurine to be placed alongside other characters or in the various "fighters." Life-size models even exist in LEGO stores or at special events, but no instructions on how to make one at home exist. The job is not impossible, as there have been people who have successfully created varying sizes of the droid themselves.
Collect as much reference material on R2D2 that you can. As no plans exist, you are going to have collect many photos of the droid. Luckily, as it is very popular, this will be easy. There are also many Star Wars reference books such as the "Star Wars Essential Guide to Droids" that can help you.
Organise all of your pieces first into colours and then into size and type.
Plan out on paper how you are going to work the various designs on the body and head.
Begin assembling the legs first as they are the easiest part and have the least amount of detail. Do not try to make the model completely solid. Eric Harshbarger, a Lego modeler who has created a life size R2D2, only made the legs one stud thick.
Begin working on the head. Note all of the details from your reference material and plans. This is where you are going to use most of the grey bricks.
Begin assembling the body. Eric Harshbarger mentioned that he made his life-size model body two studs thick. Adjust accordingly for your size. Pay close attention to your reference material and plans.
Put all of the completed body parts together starting with the head and body. Attach the legs last.
If you plan on making a larger R2D2, create a smaller version first in order to "get a feel" for the project.