How do I make actual use an IPv4 block that I own?

There are two things you need:

First you need an ISP that will act as the sponsoring LIR for you. Their role is just book keeping and maintaining the contractual chain between you and RIPE NCC.

Then you'll need an ISP that will route your addresses and announce them to the rest of the world using BGP.

Those two functions can be provided by a single ISP but it doesn't need to be.

Usually ADSL and VDSL providers only offer services using their own addresses. You'll need an ISP that specialises in business solutions. Such an ISP will probably be more expensive than the regular ones. I'm unfortunately not familiar enough with the French market to advise you on that.

An alternative might be to find an ISP that will create a tunnel for you. The routing will not be optimal and there will be a little overhead, but it might be cheaper. Technologies used for the tunnelling are things like LISP and GRE. LISP has the advantage that it can provide redundancy over multiple connections and that you can move your addresses around any way you like.

How about returning it.

The usage of a complete /24 for just a few servers is a waste of precious IP space in a world of shortage.

This does not mean you do not need or even 'deserve' some address space! Just ask your local provider to route some more to your connections, maybe a block of 8 or even 16 addresses would mostly suffice.

PI (Provider Independent) space would be useful if you have a multiple uplinks and you are able to announce it yourself. This would provide redundancy but in your question emphasized more on the use of public addresses than redundancy.