I will start by saying I am not a member of the IStructE. I did intend to go for it once so looked into it, but have since changed industries.
Assuming you can demonstrate you have a suitable academic background (you may have to submit a report outlining your qualifications and experience for review) you will be allowed to apply for the Part 3 membership exam.
The exam is a seven hour open book paper and generally includes the following:
Outline design for two distinctly different structural solutions. One in concrete, one in steel is no use. You would need to something like a braced frame and solid core/shear walls etc. The outlines need to present sufficient calculations to show they are feasible.
You then select one of the designs to take forward. You must do the calculations to size all the primary members, foundations, walls, columns, floors, beams etc.
Then there will be a change to your original brief so you must modify your design to suit, with supporting calcs as necessary.
You will then be required to do drawings (to a standard you would issue if at work) showing the main structure and details. I believe rough sketches are frowned upon.
Then there will be something along the lines of specification writing and technical letters to the client to justify the change following the scope change.
It is a lot of work to do in one day and the pass rates are usually pretty low at around 35%. I know a more people who have failed than have passed.
They format of the exam is usually pretty simililar. If I remember correctly there is usually 2 steel questions, 2 concrete questions, 1 bridge question, sometimes a timber question, and then the more specialist questions which occur from time to time. Things like reservoirs, pumping stations, topside offshore, subsea offshore etc.
In my opinion the questions can be hit or miss. If you get a question similar to something you have done in the past then you are OK. If you have no direct experience for any of the questions you might as well give up.
After each set of exams the examiners publish a report where they discusss the questions, the expectations, and then what was actually submitted. It makes interesting reading if you plan to take the exam.