I top off (trim the nice top leaves and stem off) my stem plants and replant that top cutting right away. It has no roots since it's just the top part of the stem and leaves. If you are able to anchor your stems in the substrate, then go ahead and plant them. This stem doesn't need to have roots in order to be planted in the substrate, so just plant them like normal. It will grow roots pretty quickly. What kind of substrate do you have? You shouldn't have to tie them down - the stems should stay in place. I had regular aquarium gravel for awhile before I switched to Eco Complete, but even the gravel wasn't too hard to plant the stems in - they stayed anchored.
You can float them if you want to - stem plants will grow roots from the middle of the stem too that can become a new plant. But it's not really necessary. If planted, the end in the gravel will develop roots in about a week.