This is a general question. you haven't told me how method do you want to split your data? do you want to split them randomly (from first, middle, and end of the dataset) or you want to use the first part as train and end part as a test?
Anyway, You don't need to do this manually, Matlab can automatically divide your data set in which way you want to split (random or not random). You can get more information Here. Or type:
help nndivide
You can choose the function do you want then using it in the code. For example, I want to split my data according to which way I want (first part for train and middle for validation and end part for test). So first I split my data in the indTrain, indVal, indTest by index them, then using divideind.
% For a list of all data division functions type: help nndivide
net.divideFcn = 'divideind'; % Divide data %Divide targets into three sets using specified indices%
net.divideMode = 'sample'; % Divide up every sample
net.divideParam.trainInd = indTrain; % 70% of the data set was used for train
net.divideParam.valInd = indVal; % 15% of the data set was used for validation
net.divideParam.testInd = indTest; % 15% of the data set was used for test
