I've had a bit of a break from PCM and I am finding it quite confusing. I can't seem to find a guide or tutorial about it. I've got some questions below
1) Are dossiers only used to increase the likelihood of a rider signing for you?
2) At what percentage is it likely that a rider will sign for you?
3) Do you have to open a Dossier to sign or re-sign a rider?
4) How many points do you get throughout the season?
5) Do you get the points back to assign to another rider once the rider has signed for you?
I'm not totally sure about some of these, for a while I've only signed riders with dossiers at 100%. I think you can do without but their wage demand will increase, quite drastically possibly, if the interest is low.
You end up with twice as many points as you start with, I think that's 30/60 across the board, with a few points added twice a month. So your first targets should be those with low interest, the interest increases twice per month after you opened a dossier. But it doesn't rise equally fast for all riders, you might well not get close to 100%. But the points are gone, you can't re-assign them to another rider.
Crucially, once you reach the 1st of August, you reach the signing stage. You get back to (or another, whichever way you look at it) 60 points immediately. To enter negotiations, you will have to spend those points. It's very easy to waste points by opening negotiations with riders you cannot afford and end up without a whole team.
The good news is, unlike in older editions, you can sign riders immediately, no more waiting to see if they will accept or not. However, just because you reach an agreement you haven't signed them yet, that confuses people. Instead, a new email in your inbox is generated and you have to confirm the signing. When you do that, your budget gets reduced accordingly. If you reach a lot of agreements with a low budget your money soon runs out and you cannot confirm the contracts anymore and thus not sign the riders.
Basically, make a plan on the 1st of May and stick with it. Also, if you have a WT team then there can be riders up to an AVG of 73 or so (usually from your main country) you can sign without spending points and you cannot open dossiers for those. I recommend searching for those riders with the old search function and shortlisting any you like. Obviously, they'll still want a wage, they just don't cost negotiatrion points.
I recommend making a backup of your save game on the 1st of August, at least until you get used to the new system.