How to apply solder paste with stencil in a 2 sided PCB?
Put a spacer all around underneath the edge of the board thicker than the layer of components on the bottom layer of the board, and bind a layer on top of it to secure the board in place (or fabricate it from a single piece in the first place). Make sure that the top of the holding layer and the top of the board are even to make spreading the solder paste easier.