Fix

Read a good primer on the subject. Examples are:

1. All about model aircraft. Peter Chin.

2. Radio Controlled Model Aircraft. David Boddington.

3. Build from magazine article that has step by step guide for beginners, RCM&E for example which also has a very active forum where further help can be gained.
www.modelflying.co.uk/

4. Buy a reputable beginners kit that has detailed step by step building guide. There are online forums where extra help can be gained. Some examples:
RCM&E www.modelflying.co.uk/
RCMF www.rcmf.co.uk/4um/index.php
RC Groups www.rcgroups.com/united-kingdom-344/
RC Universe www.rcuniverse.com/forum/forum.php?fh=1

Fix

1. Use a protective transparent covering paper to protect the plan, and build on to that.

2. Make a copy of the plans by tracing or scanning (ensuring a 1:1 scale)

Comment

The best transparent covering paper we have come across for this is the backing sheet to Solarfilm or Solartex or thin plastic decorators dust sheets.

Fix

Use a jig to ensure the fuselage build is true, i.e. without warps.

Comment

While jigs are available commercially, it is sometimes necessary for the modeller to make his own. That said, the common requirement is to fix a series of set squares to the plan so that the two fuselage sides can be assembled and glued in perfect alignment, It is often necessary too, to dry assemble first, to doubly ensure that when assembled the alignment is spot on. See below for a superb example of a custom built fuselage jig from aeromodeller Juergen BE

Fix

Adapt the construction to suit the tank before commencing building.

Comment

If a fuel pump is not used, the tank needs to be close to the firewall, i.e. just behind the engine so that the carburettor can easily draw fuel against gravity in all flight attitudes. Should a tank be needed that will not physically fit into this space then this indicates that something is either wrong with the plan design, or there is a builder error, e.g. tank and engine too large for the model. Conversely, a  builder wanting longer flight times, or adapting another power source for IC, may have made this decision, in which case fuselage design may have to be modified to take the larger components.

That said, minor plan design errors can and do occur and if these are not picked up at the beginning of a build, then the builder can have have problems later when tank removal is needed. The point is here do not assume all will be well, i.e. make sure you will be easily able to remove the the fuel tank for servicing needs when required, even if this means making making small adjustments to the plan's former locations for example.

Fix 1 The Lavoie method.

Use the Lavoie method.

Comment

The Lavoie method for building longeron based fuselage structures uses blocks and pins and special home made tools. The system costs nothing and is a brilliant way to build. The method can be easily adapted for use in many areas of building. See the attached videos.

Fix 2 Traditional Pinning.

Where components have substantial girth pinning directly has little to no effect. That said all joints must be tight so pressure is typically applied during the pinning.

Comment

Pinning through the components can still be avoided if so desired either by using the Lavoie method with blocks or by pinning outside longerons etc. while applying pressure.

Photo shows direct pinning of substantial tailplane components.

Fix 3 Use a Magnetic Building Board. Cr. A Morris

Magnetic building boards with low cost ceramic magnets offer a really good solution which is non destructive. To get started on this idea see the embedded video from Carl Bayer.