The smooth way to tell your partner they are TERRIBLE IN BED
Well, you definitely don't have/want to tell them that they are horrible in bed. If I were in your position, I would just ask if it's okay if I shared some of the things that really do it for me, then propose that we spend a while in bed, playfully exploring and practicing those things during the weekend/a day off.
Take a playful approach to it. Don't put too much pressure on them to get it right straight away, give them plenty of feedback and guidance, keep the mood light, etc. The are probably not going to be upset about spending a day in bed exploring your body and learning how to really get you there if they really love you and care about whether or not you are enjoying yourself.
On the flip side, you need to show just as much willingness and interest in what they would like you to do for them in bed. Like I said, take a day to do some playful exploration of each other's bodies and communicate to each other what is working and what needs to be worked on, and don't take any slight criticisms or corrections to heart. Figuring each other's bodies out can be a fun and playful experience, depending on how a couple goes about it. If you don't say or do something to make them aware of the fact that they are not a great lay, then what incentive do they have to work on it?
If you tell them it was good, then that's what they are going to believe, so you can't be too upset about them strutting around being pleased with themself when you were the one that told them that, even though they didn't.
Another option is to have it rigged. Choose the sexual instruction book from these reads for men here and/or reads for women here about the specific area you want them to improve, then suggest to read it together. Your job is to then emphasize how understated something is whenever that something is mentioned. This will enable you to be very critical and straight to the point without actually doing so because the manual is doing it for you
Whatever you do, do not be directly overly critical of their performance in bed. Could be a sore spot for most people and can end up damaging the performance more than help it progress.
Anyway to help them improve here is an encyclopedia of techniques through each one of these guides, ENJOY!
