You will need a tablespoon ("tbsp"), a teaspoon ("tsp"), kitchen scales, knife(s), spatula, potatoe peeler, small liquid measure (I use a washing powder scoop!), food whisk (electric), food processor (electric), 21cm dia x 10 cm mixing bowl, 1 pint measuring jug, muffin tray, long match stick and wire rack.
Pre-heat the oven to 175°C and place 12 muffin cases into a muffin tray. Measure out the solids into a large mixing bowl and blend well together. Measure out the liquids into a separate container (e.g. a 500ml jug) and whisk well together. Add most (but not all) of the whisked liquids to the dry ingredients, stirring whilst so doing, to make a fairly firm mixture. I usually have 1.5 fl.oz. of liquid left at this point which will later be discarded but is available in case the mixture is too firm after the apple is added.
Either (a) peel, core and dice the apples into approx 5mm cubes (preferred), or (b) peel the apples, slice into 6 segments, remove top, core and tail from each segment and then reduce them in a food processor, or alternatively (c) leave the peel in place and simply segment, top, core and tail and treat in the food processor.
Add the processed apples to the muffin mixture and stir in well. Measure out into the 12 muffin cases (each will probably take about 1.5 tablespoons of the mixture) and bake in the oven for 25-30 minutes until cooked. A matchstick inserted into the centre of a muffin at this stage should not be wet or sticky/tacky when withdrawn. Remove cooked muffins from the muffin tray and allow to cool on a wire tray.
Option (a) leads to muffins containing 'bits' of apple like chocolate chip muffins, while option (b) produces a more homogeneous muffin but avoids the cosmetically unsatisfactory phenomenon of certain types of apple acquiring a brown colour once peeled and exposed to air prior to blending with the mixture, whereas option (c) cuts air exposure to the minimum and is quicker than (a) or (b) as it avoids the peeling process. It may well appeal to those who believe the skin contains valuable extra nutrients. I have tried all three options and prefer option (a) over the others.
I find that with apples, if nothing else, the components of the food processor can just be rinsed immediately after use under the tap and placed in the drying rack to drain and dry and do not really require washing with detergent at all.
The muffins can be left on the cooling rack overnight, perhaps covered with a clean cloth, but thereafter need to be stored in an airtight environment lest they dry out before consumption.