You can simply grind the plant material (target plant part) in water/alcohol.

Solvent to sample ratio of 10:1 (v/w) solvent to dry weight ratio would be ideal .

You can also go for other methods { http://www.ipharmsciencia.com/Dacuments/1/11.pdf }

The root and branch of the test plant were well dried and then grinded to powder. About 200g of the powder were separately soaked in 400ml of (1)95% Ethanol, (2)Distilled water and (3)Ethyl acetate in a 500ml reagent bottle and stoppered. This was allowed to stand for 14 days to permit full extraction of the active ingredients. The fluids were then filtered using Whatman No1 filter paper. The extracts were rotary dried to obtain the concentrate. It was then kept in fridge prior to use. A 2.0g/l solution of each extract was prepared and fractionated into 0.4g/l, 0.2g/l and 0.1g/l concentrations needed for the bioassay

Simple way is just shade dry the materials and powder it. Boil the powder with desired volume of distilled water and filter and lyophilize the filtrate. Once you get dried extract wash the extract repeatedly with hexane or petroleum ether to remove fat from extract. Or else before aqueous extraction also you can defat the plant material by soxholet extractor.

Mentha species can be extracted easily in water - as infusions/teas. Alcohol isn't truly, in my experience necessary, for this highly useful herb. However - a 1:5 ratio marc: men strum in 45% alcohol would be sufficient 

In Dental Herbalism: Natural Therapies for the Mouth (2014 Healing Arts Press), chapter 11 contains a materia medica of over 40 herbs & chapter 12 is devoted entirely to various delivery media/recipes. Recipes are also scattered throughout the text.

 Mentha species are rich in essential and volatile oils. if your idea is to have total content of the material without loosing volatile components you can go for cold maceration where water and your dried powder can be kept in a closed container at room temperature with intermittent shaking for 24 hours. you can also perform extraction with increased polarity where alcohol will be the first solvent followed by water. double distilled water will be preferable. 

I will refer you to the work of ANKIT GUPTA, MADHU NARANIWAL & VIJAY KOTHARI titled "MODERN EXTRACTION METHODS FOR PREPARATION OF BIOACTIVE PLANT EXTRACTS"

International Journal of Applied and

Natural Sciences (IJANS)

Vol.1, Issue 1

use any of the conventional methods as stated.

or the work of Odey et al. (2012) on Preparation of Plant Extracts from Indigenous Medicinal Plants.

International Journal of Science and Technology, 1(12); 2012

@ Oludare Temitope Osuntokun

How did you soak in water for TWO WEEK!!!!?

Won't there be decays, spoilage and transformation of the whole content to all sorts of parasites?