For Reddit, you need to join and be active in subreddits related with your niche.
You can also create a subreddit in your niche, and then make some crossposting to/from other subreddits.
For Twitter you need to engage in recent threads started by popular accounts in your niche.
Make useful replies and people will start noticing you.
All this among posting 2-3 tweets/day with useful content in your niche.
About monetizing, you need to have a traffic funnel from the account profiles to your site or other platform where you sell your stuff.
For this to be efficient, just customize the profiles with a short and sweet biography about what is that you offer, and make sure you have a CTA (call to action) that directs visitors to your money site/other platform.