Pokemon Blue Softlock Marathon Guide

Start a new game and walk outside
Walk right until the girl is on the left edge of the screen, then walk right with her keeping her on the edge to force her to walk right.
Once she's in the column left of the sign, go right and up around the house to put her on the tile one up from the bottom left of the screen.
Once she goes up a tile from there, walk left and around her to below her and walk up with her until she reaches the grass.
Once she reaches the grass, go into the grass tile on the right of her to trigger the oak cutscene, which should lock you on top of Oak's lab.

Shen's original example video
https://youtu.be/MWymFzb5RzE?si=N0I4TbABeGE2IezE