Order a stencil for the board from exporting out the gerber paste layer from Diptrace. oshstencils.com is good, but larger stencils can be made in steel cheaply in China too.

Use Diptrace to print for toner transfer etching. Use preview for alignment and set mirror and print in black only options. Select the pads, holes, traces, copper pours, and board outline. For instructions on etching: https://medium.com/@cinlin/diy-flexible-circuit-boards-58c3794ed0cb

Apply low temperature or leaded solder paste (LEDs do not like 250'C) with the stencil, populate with proper ESD precautions, reflow with an oven.

https://medium.com/@cinlin/soldering-smd-components-on-flexible-pcb-b0b2f6c3de21