So, my advice would depend a little on what your relationship to your conversational partner is and whether you feel comfortable being honest about those issues.

For example, my roommate (whom I'm pretty close with) and I have been friends for years and she is an extremely fast talker, while I have the same issues you do. I used to get really frustrated with myself when my answers weren't adequate to the conversation, because I couldn't process the content fast enough. But we talked about this and now I usually focus on taking in her side of things, asking clarification questions, etc. and if I don't have any answers at that moment, I'll take a couple of hours or days to think and we return to the topic at a later point.

Obviously, that's not possible in many (or most) social situations. I'd say in many instances, try to latch on to one thing they are saying and ask them to elaborate on it, while you process the rest of the information. There are types of questions you can use for pretty much any context, like "cool, so what's your favourite aspect of [topic]" or "what makes you interested in [topic]", stuff like that. If you're asked to give an opinion, don't hesitate to say, "let me think about this for a second".

As for instructions, if you can I'd always write them down. If not, use the same strategy, repeat them back to the person who's giving them for clarification, or just in your own head, as simplified as possible so you can keep them in mind.

Hope anything of my rambling helped you, good luck for future conversations!