I've done truffles every visit to Amsterdam (5 now), with mushrooms also on the first visit before the law changed
The advice I've gotten every time from the shops is to eat them on an empty stomach, and also if you're unsure how they will affect you, to eat half first, wait an hour, then eat the rest (or don't - if you don't like the effect or feel it's strong enough already).
I trip with my boyfriend and we usually buy two boxes, share one and then open the other an hour later. We have breakfast and a smoke in the morning, then wander about until lunchtime and have the truffles for lunch. It's great to have food handy for after the mushrooms have kicked in, you might not feel like eating, but chocolate or fruit are great fun when you're tripping. Oranges / orange juice have vitamin C which can help calm the effects of the trip if it's too intense, and chocolate is supposed to enhance the experience. I have memories of two different trips and fruit..
The first was eating an orange from albert heijn on a hotel bed watching the simpsons. It was so much juicier than the ones we get in the UK and I was making a hell of a mess trying to peel it open and get the segments. The juice was running through my fingers and the citrus smell was intense, after eating about half the orange I was in fits of giggles watching the TV and had to put the rest in the bin because I was such a mess and had to go wash my hands. Came back and lay down on the bed and laughed for 2 hours, couldn't move or open my eyes.
Second memory is of having berries from albert cuyp market, a different trip when we stayed in an apartment, I was sat with the raspberries and it took me about 5 minutes to eat each one... putting it in my mouth and feeling the fuzziness on the outside, then taking great pleasure in popping each little juice-filled cell individually with my tongue
I have no idea why this was so much fun, but it felt great and I was finding it hilarious, kept trying to explain to my BF why it was the best thing ever lol.
My experiences with truffles have ranged from falling asleep (did them too late at night after smoking all day, pointless lol), losing myself in my own thoughts, hysterical crying with laughter, unable to move for extended periods of time, to mild-medium visuals - clouds and shadows are great for this, things that move on their own or aren't completely solid. Amsterdam rooftops also, spent ages feeling like I was in some MC Escher art looking out over the uneven angled rooftops and church towers!
With regards to smoking weed, try and hold off as long as you can, until the truffles start to kick in, before getting completely baked. Seems to us the more stoned we are beforehand, the less effect the truffles have. But smoking a joint after the psilocybin takes effect really seems to enhance the experience, especially visuals and laughing.
Although as the above poster says, focus sort of goes a bit, so you might want to spend the time waiting for the psilocybin to kick in, rolling a few joints for when you are worse for wear later on
Once you've started get them down as quick as possible and straight afterwards go and brush your teeth (or chewing gum if you aren't near a sink)