io layout
it is a separate and very specific area of IC design. You have to solve beyond pure functionality design task an issues dealing with ESD and Latch-up protection. If it is a learning task, you have to concentrate on basics. Chances to implement robust I/O cell for first time I assume <30%.
Good books could help, e.g. Dabral/Maloney "Basic ESD and I/O Design", ... Also it is desirable to have a look at the layout samples from relatively free foundry's I/O libraries, say TSMC.