It is difficult to give a boilerplate plan, as each plan would need to
be tailored to a greater or lesser extent to meet the needs of each
individual case and the methodologies used.  Do you have any PM
experience?  If not, depending on the size and complexity of the
project, you may find that you have been thrown in the deep end.  In
that case I would get PM training pronto, before you end up repeating
the mistakes others have made before you.

Frankly, your asking about this would lead me to conclude that you should
not be doing this. However, good luck.

I take it you mean a plan to develop a database,  and a related application.
If you really mean a projec tto develop a new DBMS,  then please correct me.

What's worked for me is this:

Conceptual (or data analysis) Phase:

Try to group all the values the database will make available into
ATTRIBUTES.
two values belong in the same attribute if they describe the same aspect of
the subject matter.  Connect each attribute to the subject matter by
specifiying what ENTITY or RELATIONSHIP the attribute describes.  For each
attribute, come up with a DOMAIN,  the set of values the attribute can take
on.  Don't get lost in details.  For now,  whether a domain is CHAR(20),
CHAR(30) or VARCHAR can probably be deferred.

If possible, learn how each entity is identified in the real world.  If
possible, identify KEY ATTRIBUES that are suitable for identification
purposes.

Logical  Design Phase.

Group the attibutes you collected in the last phase into RELATIONS.  An
attribute can participate in more than one relation, but be careful.  Each
relation should assert something meaningful about the real world described
by the attributes.

Use normalization to determine whether your set of relations is minimal or
not.  Don't worry about performance at this stage.

Use FOREIGN KEYS to implement relationships.  Use KEY Attributes to identify
entities.  Use surrogate keys where appropriate.

Phyical design phase:

Take into account the logical design from the previous phase,  and consider
as well such things as the particular RDBMS you will use, the operating
system and the resources available,  the volume of data,  the load imposed
by access to the data,  and acceptable response times.

If you were hazy about domains in the conceptual phase,  clarify them now.

If you need to denormalize for reasons of performance or convenience, do so.
But don't just denormalize.  Adopt some other design principle that will
make the whole database hang together.  Otherwise, the pieces will be hanged
separately.  :-)

If you are using an SQL interface,  represent relations with tables and
indexes.

Make your design sound and simple,  but not too simple.

Turn your design into a build script and build a database.
By this time,  the application developers are waiting for you.

The above is oversimplified,  but what do you want?