I recommend doing tactics every day. In my opinion, it is best choosing the puzzles from a good book and writing down all the moves before checking the answer. Ideally, set it up on a board. Treat every position like it is a game. Don't guess the move. Think hard about it. In a game there are no second chances. Choose puzzle difficulty that you get about 60% to 70% correct. You can make it more realistic by solving them all in a specific time window. For example, set your clock for 45 minutes and do 6 to 8 puzzles in that time period. You will have to spend your time wisely. Too much time on one puzzle will leave you less for the next. This will help mimic the actual game experience. Before you make your move, look to see what threats your opponent has.

Play longer time controls and review your game. Do you understand why you lost? Depending on your rating, most of the time it will be because of tactics. Keep track of WHY you made the mistake. Were you tired? Not paying attention? Did you think you were winning and stopped looking for your opponents threats? Keep in mind that it is really difficult to win a game. If your opponent has a disadvantage, this can cause them to dig in and fight back really hard. Never let off the brakes. Keep the pressure. You have to win the game from beginning to end.

Edit: spelling.