You need to mix both parts of epoxy thoroughly for it to cure properly.
Always mix on a non-absorbent surface. If the surface absorbs one part more than the other, it will throw off the resin-to-hardener ratio and ruin the bond.
I like to cut up plastic containers from groceries into small mixing trays perfect for each project.
Never mix epoxy on paper or cardboard. These can absorb the mixture and affect the result.
Once applied, epoxy may appear dry but still needs up to 24 hours to fully cure.
I leave the mixing stick on the leftover glue if the stick hardens and lifts off the plastic cleanly, I know the mix was correct.
Cleanup tips:
Alcohol or acetone can remove uncured epoxy, but be cautious they may also strip dye or finishes.
Epoxy doesn't stick to packing tape, so you can use it to mask off areas safely.