The risk really drops at 4 months, but I co-slept from birth with both of mine and probably will with DC3. It just makes sense to me instinctually. I think it's quite likely that being close to them and in tune, the lighter sleep you both go into, the positive effect on breastfeeding etc all provide a protective effect against SIDS which probably cancels out the higher inherent risk of an adult bed, particularly if you're being careful about the risky parts of an adult bed, which are gaps, bedding, suffocation, overlying and overheating.
In a c-shape around baby (similar to recovery position) with their head level with your boob.
Baby on their back, or side facing you.
My lower arm as a barrier between baby and pillow.
Using a bedside cot (I converted one and then bought a purpose built one for DC2) so they could not roll out of the bed.
Duvet behind my back, and up to my knees (knee was a barrier between blanket and baby). Cellular blanket over baby and me. No sleeping bag (Apparently you can do it with a sleeping bag, I just always felt they got too hot.) Don't swaddle. I wore warm PJs (Autumn babies) and they wore a vest and sleepsuit - just dress for the temperature. I went by the rule of feeling their chest or back and if it was sweaty remove a layer, if it feels cool put one on.
No smokers - smoking + co-sleeping is quite risky apparently. I occasionally smoke socially but absolutely not when co-sleeping, I don't touch them at all.
No alcohol unless I could be sure it would have worn off by bedtime. I was quite strict about it. One glass up to about early evening was OK, but I wouldn't have drunk even a tiny amount after about 6pm, nor drunk a large amount during the day. If I did drink (very rare TBH) I put the baby in a separate cot. I didn't like those nights because it was so disturbing to keep waking up with them! So I simply didn't drink very often. It doesn't bother me at all, I'm not really a drinker anyway.