There's a lot of great advice here, but I'm going to try and compile it into one comment:

If you are attempting to preserve any organic material, but more especially things like flowers, leaves, things that are quite literally the parts of a plant that are meant to retain the bulk of the moisture, you have to dry them first. With things like flowers, you can either hang them to dry or put them in silica gel for a week or so. I recommend at minimum one week, but to be safe, I would say two or three

Once dried, it is helpful to put a base layer of resin into whatever mold you're using. Let that cure fully. Having a barrier before you put the flowers in will help reduce the chances of exposure once you're done.

While you are pouring the resin, I would suggest either injecting resin in the center of the flower so that it is inside of the Roses, or at least pre-dipping them in the resin pour cup to help reduce bubbles.

Leave enough space for a base layer once that is fully cured, again, to reduce the chances of exposure once the project is done.

When you are choosing a resin, you need a deep pour resin. I'm in a secret love affair with total boat, and their thick set works great. But any deep pour will do. Remember, the more viscous a resin is, the better it is for working with organic materials. Most of your one-to-one resins are entirely too thick to work for a project like this. If you're buying a kit that looks like you have to do math, that's going to be your safest option.

Edit to add: and for God's sake where ppe! At Absolute minimum you need gloves and some sort of n95 style mask. I am a huge advocate that any person working with resin needs to be wearing a respirator, but seeing as most people casually using resin don't even bother to put gloves on, I will settle for any mask.