The single-nucleus sci-ATAC-seq (snATAC-seq) uses the same strategy (combinatorial indexing) and oligo design as sci-ATAC-seq. The snATAC-seq had some experimental modifications to make the method work in frozen tissue samples. You can check the Supplementary Figure 1 for detailed information about experimental optimisation the authors did.
For the step-by-step library generation, it is the same as sci-ATAC-seq. Click the sci-ATAC-seq page for details.